Re: [Qemu-devel] [PATCH qemu v14 18/18] spapr_pci/spapr_pci_vfio: Support Dynamic DMA Windows (DDW)

[David Gibson]

On Thu, Mar 24, 2016 at 01:32:48PM +1100, Alexey Kardashevskiy wrote:
> On 03/23/2016 05:11 PM, David Gibson wrote:
> >On Wed, Mar 23, 2016 at 02:28:01PM +1100, Alexey Kardashevskiy wrote:
> >>On 03/23/2016 01:13 PM, David Gibson wrote:
> >>>On Mon, Mar 21, 2016 at 06:47:06PM +1100, Alexey Kardashevskiy wrote:
> >>>>This adds support for Dynamic DMA Windows (DDW) option defined by
> >>>>the SPAPR specification which allows to have additional DMA window(s)
> >>>>
> >>>>This implements DDW for emulated and VFIO devices.
> >>>>This reserves RTAS token numbers for DDW calls.
> >>>>
> >>>>This changes the TCE table migration descriptor to support dynamic
> >>>>tables as from now on, PHB will create as many stub TCE table objects
> >>>>as PHB can possibly support but not all of them might be initialized at
> >>>>the time of migration because DDW might or might not be requested by
> >>>>the guest.
> >>>>
> >>>>The "ddw" property is enabled by default on a PHB but for compatibility
> >>>>the pseries-2.5 machine and older disable it.
> >>>>
> >>>>This implements DDW for VFIO. The host kernel support is required.
> >>>>This adds a "levels" property to PHB to control the number of levels
> >>>>in the actual TCE table allocated by the host kernel, 0 is the default
> >>>>value to tell QEMU to calculate the correct value. Current hardware
> >>>>supports up to 5 levels.
> >>>>
> >>>>The existing linux guests try creating one additional huge DMA window
> >>>>with 64K or 16MB pages and map the entire guest RAM to. If succeeded,
> >>>>the guest switches to dma_direct_ops and never calls TCE hypercalls
> >>>>(H_PUT_TCE,...) again. This enables VFIO devices to use the entire RAM
> >>>>and not waste time on map/unmap later. This adds a "dma64_win_addr"
> >>>>property which is a bus address for the 64bit window and by default
> >>>>set to 0x800.0000.0000.0000 as this is what the modern POWER8 hardware
> >>>>uses and this allows having emulated and VFIO devices on the same bus.
> >>>>
> >>>>This adds 4 RTAS handlers:
> >>>>* ibm,query-pe-dma-window
> >>>>* ibm,create-pe-dma-window
> >>>>* ibm,remove-pe-dma-window
> >>>>* ibm,reset-pe-dma-window
> >>>>These are registered from type_init() callback.
> >>>>
> >>>>These RTAS handlers are implemented in a separate file to avoid polluting
> >>>>spapr_iommu.c with PCI.
> >>>>
> >>>>Signed-off-by: Alexey Kardashevskiy <address@hidden>
> >>>>---
> >>>>  hw/ppc/Makefile.objs        |   1 +
> >>>>  hw/ppc/spapr.c              |   7 +-
> >>>>  hw/ppc/spapr_pci.c          |  73 ++++++++---
> >>>>  hw/ppc/spapr_rtas_ddw.c     | 300 
> >>>> ++++++++++++++++++++++++++++++++++++++++++++
> >>>>  hw/vfio/common.c            |   5 -
> >>>>  include/hw/pci-host/spapr.h |  13 ++
> >>>>  include/hw/ppc/spapr.h      |  16 ++-
> >>>>  trace-events                |   4 +
> >>>>  8 files changed, 395 insertions(+), 24 deletions(-)
> >>>>  create mode 100644 hw/ppc/spapr_rtas_ddw.c
> >>>>
> >>>>diff --git a/hw/ppc/Makefile.objs b/hw/ppc/Makefile.objs
> >>>>index c1ffc77..986b36f 100644
> >>>>--- a/hw/ppc/Makefile.objs
> >>>>+++ b/hw/ppc/Makefile.objs
> >>>>@@ -7,6 +7,7 @@ obj-$(CONFIG_PSERIES) += spapr_pci.o spapr_rtc.o 
> >>>>spapr_drc.o spapr_rng.o
> >>>>  ifeq ($(CONFIG_PCI)$(CONFIG_PSERIES)$(CONFIG_LINUX), yyy)
> >>>>  obj-y += spapr_pci_vfio.o
> >>>>  endif
> >>>>+obj-$(CONFIG_PSERIES) += spapr_rtas_ddw.o
> >>>>  # PowerPC 4xx boards
> >>>>  obj-y += ppc405_boards.o ppc4xx_devs.o ppc405_uc.o ppc440_bamboo.o
> >>>>  obj-y += ppc4xx_pci.o
> >>>>diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> >>>>index d0bb423..ef4c637 100644
> >>>>--- a/hw/ppc/spapr.c
> >>>>+++ b/hw/ppc/spapr.c
> >>>>@@ -2362,7 +2362,12 @@ DEFINE_SPAPR_MACHINE(2_6, "2.6", true);
> >>>>   * pseries-2.5
> >>>>   */
> >>>>  #define SPAPR_COMPAT_2_5 \
> >>>>-        HW_COMPAT_2_5
> >>>>+        HW_COMPAT_2_5 \
> >>>>+        {\
> >>>>+            .driver   = TYPE_SPAPR_PCI_HOST_BRIDGE,\
> >>>>+            .property = "ddw",\
> >>>>+            .value    = stringify(off),\
> >>>>+        },
> >>>>
> >>>>  static void spapr_machine_2_5_instance_options(MachineState *machine)
> >>>>  {
> >>>>diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> >>>>index af99a36..3bb294a 100644
> >>>>--- a/hw/ppc/spapr_pci.c
> >>>>+++ b/hw/ppc/spapr_pci.c
> >>>>@@ -803,12 +803,12 @@ static char *spapr_phb_get_loc_code(sPAPRPHBState 
> >>>>*sphb, PCIDevice *pdev)
> >>>>      return buf;
> >>>>  }
> >>>>
> >>>>-static void spapr_phb_dma_window_enable(sPAPRPHBState *sphb,
> >>>>-                                       uint32_t liobn,
> >>>>-                                       uint32_t page_shift,
> >>>>-                                       uint64_t window_addr,
> >>>>-                                       uint64_t window_size,
> >>>>-                                       Error **errp)
> >>>>+void spapr_phb_dma_window_enable(sPAPRPHBState *sphb,
> >>>>+                                 uint32_t liobn,
> >>>>+                                 uint32_t page_shift,
> >>>>+                                 uint64_t window_addr,
> >>>>+                                 uint64_t window_size,
> >>>>+                                 Error **errp)
> >>>>  {
> >>>>      sPAPRTCETable *tcet;
> >>>>      uint32_t nb_table = window_size >> page_shift;
> >>>>@@ -825,10 +825,16 @@ static void 
> >>>>spapr_phb_dma_window_enable(sPAPRPHBState *sphb,
> >>>>          return;
> >>>>      }
> >>>>
> >>>>+    if (SPAPR_PCI_DMA_WINDOW_NUM(liobn) && !sphb->ddw_enabled) {
> >>>>+        error_setg(errp,
> >>>>+                   "Attempt to use second window when DDW is disabled on 
> >>>>PHB");
> >>>>+        return;
> >>>>+    }
> >>>
> >>>This should never happen unless something is wrong with the tests in
> >>>the RTAS functions, yes?  In which case it should probably be an
> >>>assert().
> >>
> >>This should not. But this is called from the RTAS caller so I'd really like
> >>to have a message rather than assert() if that condition happens, here or in
> >>rtas_ibm_create_pe_dma_window().
> >
> >It should only be called from RTAS if ddw is enabled though, yes?
> 
> 
> From RTAS and from the PHB reset handler. Well. I will get rid of
> spapr_phb_dma_window_enable/spapr_phb_dma_window_disable, they are quite
> useless when I look at them now.

Ok.

> >>>>      spapr_tce_table_enable(tcet, page_shift, window_addr, nb_table);
> >>>>  }
> >>>>
> >>>>-static int spapr_phb_dma_window_disable(sPAPRPHBState *sphb, uint32_t 
> >>>>liobn)
> >>>>+int spapr_phb_dma_window_disable(sPAPRPHBState *sphb, uint32_t liobn)
> >>>>  {
> >>>>      sPAPRTCETable *tcet = spapr_tce_find_by_liobn(liobn);
> >>>>
> >>>>@@ -1492,14 +1498,18 @@ static void spapr_phb_realize(DeviceState *dev, 
> >>>>Error **errp)
> >>>>      }
> >>>>
> >>>>      /* DMA setup */
> >>>>-    tcet = spapr_tce_new_table(DEVICE(sphb), sphb->dma_liobn);
> >>>>-    if (!tcet) {
> >>>>-        error_report("No default TCE table for %s", sphb->dtbusname);
> >>>>-        return;
> >>>>-    }
> >>>>
> >>>>-    memory_region_add_subregion_overlap(&sphb->iommu_root, 0,
> >>>>-                                        spapr_tce_get_iommu(tcet), 0);
> >>>>+    for (i = 0; i < SPAPR_PCI_DMA_MAX_WINDOWS; ++i) {
> >>>>+        tcet = spapr_tce_new_table(DEVICE(sphb),
> >>>>+                                   SPAPR_PCI_LIOBN(sphb->index, i));
> >>>>+        if (!tcet) {
> >>>>+            error_setg(errp, "Creating window#%d failed for %s",
> >>>>+                       i, sphb->dtbusname);
> >>>>+            return;
> >>>>+        }
> >>>>+        memory_region_add_subregion_overlap(&sphb->iommu_root, 0,
> >>>>+                                            spapr_tce_get_iommu(tcet), 
> >>>>0);
> >>>>+    }
> >>>>
> >>>>      sphb->msi = g_hash_table_new_full(g_int_hash, g_int_equal, g_free, 
> >>>> g_free);
> >>>>  }
> >>>>@@ -1517,11 +1527,16 @@ static int spapr_phb_children_reset(Object 
> >>>>*child, void *opaque)
> >>>>
> >>>>  void spapr_phb_dma_reset(sPAPRPHBState *sphb)
> >>>>  {
> >>>>-    sPAPRTCETable *tcet = spapr_tce_find_by_liobn(sphb->dma_liobn);
> >>>>      Error *local_err = NULL;
> >>>>+    int i;
> >>>>
> >>>>-    if (tcet && tcet->enabled) {
> >>>>-        spapr_phb_dma_window_disable(sphb, sphb->dma_liobn);
> >>>>+    for (i = 0; i < SPAPR_PCI_DMA_MAX_WINDOWS; ++i) {
> >>>>+        uint32_t liobn = SPAPR_PCI_LIOBN(sphb->index, i);
> >>>>+        sPAPRTCETable *tcet = spapr_tce_find_by_liobn(liobn);
> >>>>+
> >>>>+        if (tcet && tcet->enabled) {
> >>>>+            spapr_phb_dma_window_disable(sphb, liobn);
> >>>>+        }
> >>>>      }
> >>>>
> >>>>      /* Register default 32bit DMA window */
> >>>>@@ -1562,6 +1577,13 @@ static Property spapr_phb_properties[] = {
> >>>>      /* Default DMA window is 0..1GB */
> >>>>      DEFINE_PROP_UINT64("dma_win_addr", sPAPRPHBState, dma_win_addr, 0),
> >>>>      DEFINE_PROP_UINT64("dma_win_size", sPAPRPHBState, dma_win_size, 
> >>>> 0x40000000),
> >>>>+    DEFINE_PROP_UINT64("dma64_win_addr", sPAPRPHBState, 
> >>>>dma64_window_addr,
> >>>>+                       0x800000000000000ULL),
> >>>>+    DEFINE_PROP_BOOL("ddw", sPAPRPHBState, ddw_enabled, true),
> >>>>+    DEFINE_PROP_UINT32("windows", sPAPRPHBState, windows_supported,
> >>>>+                       SPAPR_PCI_DMA_MAX_WINDOWS),
> >>>
> >>>What will happen if the user tries to set 'windows' larger than
> >>>SPAPR_PCI_DMA_MAX_WINDOWS?
> >>
> >>
> >>Oh. I need to replace SPAPR_PCI_DMA_MAX_WINDOWS with windows_supported
> >>everywhere, missed that. Besides that, there will be support for more
> >>windows, that's it. The host VFIO IOMMU driver will fail creating more
> >>windows but this is expected. For emulated windows, there will be more
> >>windows with no other consequences.
> >
> >Hmm.. is there actually a reason to have the windows property?  Would
> >you be better off just using the compile time constant for now.
> 
> 
> I am afraid it is going to be 2 DMA windows forever as the other DMA tlb-ish
> facility coming does not use windows at all :)

Ok, that sounds like a vote for removing the property.

> >>>>+    DEFINE_PROP_UINT64("pgsz", sPAPRPHBState, page_size_mask,
> >>>>+                       (1ULL << 12) | (1ULL << 16) | (1ULL << 24)),
> >>>>      DEFINE_PROP_END_OF_LIST(),
> >>>>  };
> >>>>
> >>>>@@ -1815,6 +1837,15 @@ int spapr_populate_pci_dt(sPAPRPHBState *phb,
> >>>>      uint32_t interrupt_map_mask[] = {
> >>>>          cpu_to_be32(b_ddddd(-1)|b_fff(0)), 0x0, 0x0, cpu_to_be32(-1)};
> >>>>      uint32_t interrupt_map[PCI_SLOT_MAX * PCI_NUM_PINS][7];
> >>>>+    uint32_t ddw_applicable[] = {
> >>>>+        cpu_to_be32(RTAS_IBM_QUERY_PE_DMA_WINDOW),
> >>>>+        cpu_to_be32(RTAS_IBM_CREATE_PE_DMA_WINDOW),
> >>>>+        cpu_to_be32(RTAS_IBM_REMOVE_PE_DMA_WINDOW)
> >>>>+    };
> >>>>+    uint32_t ddw_extensions[] = {
> >>>>+        cpu_to_be32(1),
> >>>>+        cpu_to_be32(RTAS_IBM_RESET_PE_DMA_WINDOW)
> >>>>+    };
> >>>>      sPAPRTCETable *tcet;
> >>>>      PCIBus *bus = PCI_HOST_BRIDGE(phb)->bus;
> >>>>      sPAPRFDT s_fdt;
> >>>>@@ -1839,6 +1870,14 @@ int spapr_populate_pci_dt(sPAPRPHBState *phb,
> >>>>      _FDT(fdt_setprop_cell(fdt, bus_off, "ibm,pci-config-space-type", 
> >>>> 0x1));
> >>>>      _FDT(fdt_setprop_cell(fdt, bus_off, "ibm,pe-total-#msi", 
> >>>> XICS_IRQS));
> >>>>
> >>>>+    /* Dynamic DMA window */
> >>>>+    if (phb->ddw_enabled) {
> >>>>+        _FDT(fdt_setprop(fdt, bus_off, "ibm,ddw-applicable", 
> >>>>&ddw_applicable,
> >>>>+                         sizeof(ddw_applicable)));
> >>>>+        _FDT(fdt_setprop(fdt, bus_off, "ibm,ddw-extensions",
> >>>>+                         &ddw_extensions, sizeof(ddw_extensions)));
> >>>>+    }
> >>>>+
> >>>>      /* Build the interrupt-map, this must matches what is done
> >>>>       * in pci_spapr_map_irq
> >>>>       */
> >>>>diff --git a/hw/ppc/spapr_rtas_ddw.c b/hw/ppc/spapr_rtas_ddw.c
> >>>>new file mode 100644
> >>>>index 0000000..37f805f
> >>>>--- /dev/null
> >>>>+++ b/hw/ppc/spapr_rtas_ddw.c
> >>>>@@ -0,0 +1,300 @@
> >>>>+/*
> >>>>+ * QEMU sPAPR Dynamic DMA windows support
> >>>>+ *
> >>>>+ * Copyright (c) 2015 Alexey Kardashevskiy, IBM Corporation.
> >>>>+ *
> >>>>+ *  This program is free software; you can redistribute it and/or modify
> >>>>+ *  it under the terms of the GNU General Public License as published by
> >>>>+ *  the Free Software Foundation; either version 2 of the License,
> >>>>+ *  or (at your option) any later version.
> >>>>+ *
> >>>>+ *  This program is distributed in the hope that it will be useful,
> >>>>+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
> >>>>+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >>>>+ *  GNU General Public License for more details.
> >>>>+ *
> >>>>+ *  You should have received a copy of the GNU General Public License
> >>>>+ *  along with this program; if not, see <http://www.gnu.org/licenses/>.
> >>>>+ */
> >>>>+
> >>>>+#include "qemu/osdep.h"
> >>>>+#include "qemu/error-report.h"
> >>>>+#include "hw/ppc/spapr.h"
> >>>>+#include "hw/pci-host/spapr.h"
> >>>>+#include "trace.h"
> >>>>+
> >>>>+static int spapr_phb_get_active_win_num_cb(Object *child, void *opaque)
> >>>>+{
> >>>>+    sPAPRTCETable *tcet;
> >>>>+
> >>>>+    tcet = (sPAPRTCETable *) object_dynamic_cast(child, 
> >>>>TYPE_SPAPR_TCE_TABLE);
> >>>>+    if (tcet && tcet->enabled) {
> >>>>+        ++*(unsigned *)opaque;
> >>>>+    }
> >>>>+    return 0;
> >>>>+}
> >>>>+
> >>>>+static unsigned spapr_phb_get_active_win_num(sPAPRPHBState *sphb)
> >>>>+{
> >>>>+    unsigned ret = 0;
> >>>>+
> >>>>+    object_child_foreach(OBJECT(sphb), spapr_phb_get_active_win_num_cb, 
> >>>>&ret);
> >>>>+
> >>>>+    return ret;
> >>>>+}
> >>>>+
> >>>>+static int spapr_phb_get_free_liobn_cb(Object *child, void *opaque)
> >>>>+{
> >>>>+    sPAPRTCETable *tcet;
> >>>>+
> >>>>+    tcet = (sPAPRTCETable *) object_dynamic_cast(child, 
> >>>>TYPE_SPAPR_TCE_TABLE);
> >>>>+    if (tcet && !tcet->enabled) {
> >>>>+        *(uint32_t *)opaque = tcet->liobn;
> >>>>+        return 1;
> >>>>+    }
> >>>>+    return 0;
> >>>>+}
> >>>>+
> >>>>+static unsigned spapr_phb_get_free_liobn(sPAPRPHBState *sphb)
> >>>>+{
> >>>>+    uint32_t liobn = 0;
> >>>>+
> >>>>+    object_child_foreach(OBJECT(sphb), spapr_phb_get_free_liobn_cb, 
> >>>>&liobn);
> >>>>+
> >>>>+    return liobn;
> >>>>+}
> >>>>+
> >>>>+static uint32_t spapr_query_mask(struct ppc_one_seg_page_size *sps,
> >>>>+                                 uint64_t page_mask)
> >>>>+{
> >>>>+    int i, j;
> >>>>+    uint32_t mask = 0;
> >>>>+    const struct { int shift; uint32_t mask; } masks[] = {
> >>>>+        { 12, RTAS_DDW_PGSIZE_4K },
> >>>>+        { 16, RTAS_DDW_PGSIZE_64K },
> >>>>+        { 24, RTAS_DDW_PGSIZE_16M },
> >>>>+        { 25, RTAS_DDW_PGSIZE_32M },
> >>>>+        { 26, RTAS_DDW_PGSIZE_64M },
> >>>>+        { 27, RTAS_DDW_PGSIZE_128M },
> >>>>+        { 28, RTAS_DDW_PGSIZE_256M },
> >>>>+        { 34, RTAS_DDW_PGSIZE_16G },
> >>>>+    };
> >>>>+
> >>>>+    for (i = 0; i < PPC_PAGE_SIZES_MAX_SZ; i++) {
> >>>>+        for (j = 0; j < ARRAY_SIZE(masks); ++j) {
> >>>>+            if ((sps[i].page_shift == masks[j].shift) &&
> >>>>+                    (page_mask & (1ULL << masks[j].shift))) {
> >>>>+                mask |= masks[j].mask;
> >>>>+            }
> >>>>+        }
> >>>>+    }
> >>>>+
> >>>>+    return mask;
> >>>>+}
> >>>>+
> >>>>+static void rtas_ibm_query_pe_dma_window(PowerPCCPU *cpu,
> >>>>+                                         sPAPRMachineState *spapr,
> >>>>+                                         uint32_t token, uint32_t nargs,
> >>>>+                                         target_ulong args,
> >>>>+                                         uint32_t nret, target_ulong 
> >>>>rets)
> >>>>+{
> >>>>+    CPUPPCState *env = &cpu->env;
> >>>>+    sPAPRPHBState *sphb;
> >>>>+    uint64_t buid, max_window_size;
> >>>>+    uint32_t avail, addr, pgmask = 0;
> >>>>+
> >>>>+    if ((nargs != 3) || (nret != 5)) {
> >>>>+        goto param_error_exit;
> >>>>+    }
> >>>>+
> >>>>+    buid = ((uint64_t)rtas_ld(args, 1) << 32) | rtas_ld(args, 2);
> >>>>+    addr = rtas_ld(args, 0);
> >>>>+    sphb = spapr_pci_find_phb(spapr, buid);
> >>>>+    if (!sphb || !sphb->ddw_enabled) {
> >>>>+        goto param_error_exit;
> >>>>+    }
> >>>>+
> >>>>+    /* Work out supported page masks */
> >>>>+    pgmask = spapr_query_mask(env->sps.sps, sphb->page_size_mask);
> >>>
> >>>There are a few potential problems here.  First you're just
> >>>arbitrarily picking the first entry in the sps array to filter
> >>
> >>Why first?  spapr_query_mask() has a loop 0..PPC_PAGE_SIZES_MAX_SZ.
> >
> >env->sps is a nested array, 0..PPC_PAGE_SIZES_MAX_SZ-1 for base page
> >sizes, then again for actual page sizes.  You're only examing the
> >first "row" of that table.  It kinda works because the 4k base page
> >size is first, which lists all the actual page size options.
> 
> Ah. Right. So I need to walk through all of them, ok.

Yes.. well.. except that the more I think about it, the less looking
at the target page sizes seems relevant to what IOMMU page sizes can
be supported.

The fundamental constraints are that an allowed page size needs to be:
     1. Supported by the guest IOMMU
and  2. >= than the smallest available host IOMMU page size

(1) applies always, (2) is are only relevant when VFIO is in the
picture.

This is somewhat similar to the filtering we need to do for CPU page
sizes, but I don't think it's close enough that it really makes sense
to re-use the sps table.

> >>>against, which doesn't seem right (except by accident).  It's a little
> >>>bit odd filtering against guest page sizes at all, although I get what
> >>>you're really trying to do is filter against allowed host page sizes.
> >>>
> >>>The other problem is that silently filtering capabilities based on the
> >>>host can be a pain for migration - I've made the mistake and had it
> >>>bite me in the past.  I think it would be safer to just check the
> >>>pagesizes requested in the property against what's possible and
> >>>outright fail if they don't match.  For convenience you could also set
> >>>according to host capabilities if the user doesn't specify anything,
> >>>but that would require explicit handling of the "default" case.
> 
> 
> What are the host capabilities here?
> 
> There is a page mask from the host IOMMU/PE which is 4K|64K|16M and many
> other sizes, this is supported always by IODA2.
> And there is PAGE_SIZE and huge pages (but only with -mempath) - so, 64K and
> 16M (with -mempath).
> 
> And there is a "ddw-query" RTAS call which tells the guest if it can use 16M
> or not. How do you suggest I advertise 16M to the guest? If I always
> advertise 16M and there is no -mempath, the guest won't try smaller page
> size.
> 
> So - if the user wants 16M IOMMU pages, he has to use -mempath and in
> addition to that explicitely say -global spapr-pci-host-bridge.pgsz=16M, and
> by default enable only 4K and 64K (or just 4K?)? I am fine with this, it
> just means more work for libvirt folks.

So, what complicates this is the additional constraint that 1 guest
TCE map to 1 host TCE.  That constraint isn't necessary for basic,
slow, VFIO operation: VFIO_DMA_MAP on a 16M guest page will map in
multiple TCEs on a 64k pagesized host TCE table.

The kernel acceleration does rely on the host IOMMU page size matching
the guest IOMMU page size, and the kernel DDW interface sort of does,
since we directly specify the page size (qemu could request a DDW
window with a smaller pagesize than the guest pagesize, and things
should still work ok - but the guest TCE table representation would
now have multiple host TCEs backing each entry).

Allowing those more complex cases is probably more trouble than its
worth for the time being, so I think that replaces constraint (2) with
the stronger constraint:
    2a. == to one of the host IOMMU page sizes

> >>For the migration purposes, both guests should be started with or without
> >>hugepages enabled; this is taken into account already. Besides that, the
> >>result of "query" won't differ.
> >
> >Hmm.. if you're migrating between TCG and KVM or between PR and HV
> >these could change as well.  I'm not sure that works at the moment,
> >but I'd prefer not to introduce any more barriers to it than we have
> >to.
> >
> >>>Remember that this code will be relevant for DDW with emulated
> >>>devices, even if VFIO is not in play at all.
> >>>
> >>>All those considerations aside, it seems like it would make more sense
> >>>to do this filtering during device realize, rather than leaving it
> >>>until the guest queries.
> >>
> >>The result will be the same, it only depends on whether hugepages are
> >>enabled or not and this happens at the start time. But yes, feels more
> >>accurate to do this in PHB realize(), I'll move it.
> >>
> >>
> >>>
> >>>>+    /*
> >>>>+     * This is "Largest contiguous block of TCEs allocated specifically
> >>>>+     * for (that is, are reserved for) this PE".
> >>>>+     * Return the maximum number as maximum supported RAM size was in 4K 
> >>>>pages.
> >>>>+     */
> >>>>+    max_window_size = MACHINE(spapr)->maxram_size >> 
> >>>>SPAPR_TCE_PAGE_SHIFT;
> >>>
> >>>Will maxram_size always be enough?  There will sometimes be an
> >>>alignment gap between the "base" RAM and the hotpluggable RAM, meaning
> >>>that if everything is plugged the last RAM address will be beyond
> >>>maxram_size.  Will that require pushing this number up, or will the
> >>>guest "repack" the RAM layout when it maps it into the TCE tables?
> >>
> >>
> >>Hm. I do not know what the guest does to DDW on memory hotplug but this is a
> >>valid point... What QEMU helper does return the last available address in
> >>the system memory address space? Like memblock_end_of_DRAM() in the kernel,
> >>I would use that instead.
> >
> >There is a last_ram_offset() but that's in the ram_addr_t address
> 
> What do you call "ram_addr_t address space"? Non-pluggable memory and
> machine->ram_size is its size?
> 
> 
> >space, which isn't necessarily the same as the physical address space
> >(though it's usually similar).
> 
> 
> I looked at the code and it looks like machine->ram_size is always what came
> from "-m" and it won't grow if some memory was hotplugged, is that correct?
> 
> And hotpluggable memory does not appear in the global ram_list as a RAMBlock?
> 
> 
> >You can have a look at what we check
> >in (the TCG/PR version of) H_ENTER which needs to check this as well.
> 
> Ok, thanks for the pointer.
> 
> 

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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