Re: [Qemu-devel] [PATCH] intel-iommu: optimize nodmar memory regions

[Paolo Bonzini]

On 13/03/19 10:43, Peter Xu wrote:
> Previously we have per-device system memory aliases when DMAR is
> disabled by the system.  It will slow the system down if there are
> lots of devices especially when DMAR is disabled, because each of the
> aliased system address space will contain O(N) slots, and rendering
> such N address spaces will be O(N^2) complexity.
> 
> This patch introduces a shared nodmar memory region and for each
> device we only create an alias to the shared memory region.  With the
> aliasing, QEMU memory core API will be able to detect when devices are
> sharing the same address space (which is the nodmar address space)
> when rendering the FlatViews and the total number of FlatViews can be
> dramatically reduced when there are a lot of devices.
> 
> Suggested-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
> ---
> 
> Hi, Sergio,
> 
> This patch implements the optimization that Paolo proposed in the
> other thread.  Would you please try this patch to see whether it could
> help for your case?  Thanks,

Yes, this looks great.  Sergio, if you have time to test it it would
be great.  With this patch, we switch between few big flatviews at
boot (before IOMMU is loaded) or with iommu=pt, and many small flatviews
after IOMMU is loaded and iommu!=pt.  Both should be fine for performance,
and in particular the first should give no penalty at all compared to
no IOMMU.

I only made a very small change to give different names to the various
-dmar regions, and queued the patch:

diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
index f87b1033f6..e38c27e39c 100644
--- a/hw/i386/intel_iommu.c
+++ b/hw/i386/intel_iommu.c
@@ -2947,7 +2947,7 @@ VTDAddressSpace *vtd_find_add_as(IntelIOMMUState *s, 
PCIBus *bus, int devfn)
     vtd_dev_as = vtd_bus->dev_as[devfn];
 
     if (!vtd_dev_as) {
-        snprintf(name, sizeof(name), "vtd-as-%02x.%x", PCI_SLOT(devfn),
+        snprintf(name, sizeof(name), "vtd-%02x.%x", PCI_SLOT(devfn),
                  PCI_FUNC(devfn));
         vtd_bus->dev_as[devfn] = vtd_dev_as = 
g_malloc0(sizeof(VTDAddressSpace));
 
@@ -2983,9 +2983,10 @@ VTDAddressSpace *vtd_find_add_as(IntelIOMMUState *s, 
PCIBus *bus, int devfn)
          * region here just like what we've done above with the nodmar
          * region.
          */
+        strcat(name, "-dmar");
         memory_region_init_iommu(&vtd_dev_as->iommu, sizeof(vtd_dev_as->iommu),
                                  TYPE_INTEL_IOMMU_MEMORY_REGION, OBJECT(s),
-                                 "vtd-dmar", UINT64_MAX);
+                                 name, UINT64_MAX);
         memory_region_init_alias(&vtd_dev_as->iommu_ir, OBJECT(s), "vtd-ir",
                                  &s->mr_ir, 0, memory_region_size(&s->mr_ir));
         memory_region_add_subregion_overlap(MEMORY_REGION(&vtd_dev_as->iommu),

Paolo