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Re: [Qemu-devel] [PATCH V2] virtio-pci: reduce modern_mem_bar size

From: Marcel Apfelbaum
Subject: Re: [Qemu-devel] [PATCH V2] virtio-pci: reduce modern_mem_bar size
Date: Wed, 7 Sep 2016 22:51:52 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.1.1 
On 09/07/2016 06:05 PM, Michael S. Tsirkin wrote:

On Wed, Sep 07, 2016 at 06:02:25PM +0300, Marcel Apfelbaum wrote:

Currently each VQ Notification Virtio Capability is allocated
on a different page. The idea is to enable split drivers within
guests, however there are no known plans to do that.
The allocation will result in a 8MB BAR, more than various
guest firmwares pre-allocates for PCI Bridges hotplug process.

Reserve 4 bytes per VQ by default and add a new parameter
"page-per-vq" to be used with split drivers.

Signed-off-by: Marcel Apfelbaum <address@hidden>


Looks good to me, I'll queue it up.




Thanks!

BTW, RE - question about the division added to virtio_pci_notify_write,
if is actually translated by gcc as a shift:

The answer is no:

00000000000038c1 <virtio_pci_notify_write>:
    ...
    393b:       e8 74 d5 ff ff          callq  eb4 <virtio_pci_queue_mem_mult>
    3940:       48 63 c8                movslq %eax,%rcx
    3943:       48 8b 45 d0             mov    -0x30(%rbp),%rax
    3947:       ba 00 00 00 00          mov    $0x0,%edx
    394c:       48 f7 f1                div    %rcx

                                      ^^^^^^^^

    394f:       89 45 ec                mov    %eax,-0x14(%rbp)
    3952:       81 7d ec ff 03 00 00    cmpl   $0x3ff,-0x14(%rbp
    ...

Should we do something about it?
Thanks,
Marcel




---
Hi,

v1 -> v2:
  - Use inline function instead of a new proxy field (Michael)
  - Add comment explaining PCI BAR regions must be powers of 2 (Michael)

To be applied on top of:
    [PATCH v5 1/2] pc: Add 2.8 machine 
(http://patchwork.ozlabs.org/patch/666812/)

Thanks,
Marcel


 hw/virtio/virtio-pci.c | 22 +++++++++++++++-------
 hw/virtio/virtio-pci.h |  5 +++++
 include/hw/compat.h    |  6 +++++-
 3 files changed, 25 insertions(+), 8 deletions(-)

diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
index 755f921..c7610ea 100644
--- a/hw/virtio/virtio-pci.c
+++ b/hw/virtio/virtio-pci.c
@@ -294,6 +294,12 @@ static void virtio_pci_ioeventfd_set_disabled(DeviceState 
*d, bool disabled)

 #define QEMU_VIRTIO_PCI_QUEUE_MEM_MULT 0x1000

+static inline int virtio_pci_queue_mem_mult(struct VirtIOPCIProxy *proxy)
+{
+    return (proxy->flags & VIRTIO_PCI_FLAG_PAGE_PER_VQ) ?
+        QEMU_VIRTIO_PCI_QUEUE_MEM_MULT : 4;
+}
+
 static int virtio_pci_ioeventfd_assign(DeviceState *d, EventNotifier *notifier,
                                        int n, bool assign)
 {
@@ -307,7 +313,7 @@ static int virtio_pci_ioeventfd_assign(DeviceState *d, 
EventNotifier *notifier,
     MemoryRegion *modern_mr = &proxy->notify.mr;
     MemoryRegion *modern_notify_mr = &proxy->notify_pio.mr;
     MemoryRegion *legacy_mr = &proxy->bar;
-    hwaddr modern_addr = QEMU_VIRTIO_PCI_QUEUE_MEM_MULT *
+    hwaddr modern_addr = virtio_pci_queue_mem_mult(proxy) *
                          virtio_get_queue_index(vq);
     hwaddr legacy_addr = VIRTIO_PCI_QUEUE_NOTIFY;

@@ -1370,7 +1376,8 @@ static void virtio_pci_notify_write(void *opaque, hwaddr 
addr,
                                     uint64_t val, unsigned size)
 {
     VirtIODevice *vdev = opaque;
-    unsigned queue = addr / QEMU_VIRTIO_PCI_QUEUE_MEM_MULT;
+    VirtIOPCIProxy *proxy = VIRTIO_PCI(DEVICE(vdev)->parent_bus->parent);
+    unsigned queue = addr / virtio_pci_queue_mem_mult(proxy);

     if (queue < VIRTIO_QUEUE_MAX) {
         virtio_queue_notify(vdev, queue);
@@ -1609,7 +1616,7 @@ static void virtio_pci_device_plugged(DeviceState *d, 
Error **errp)
         struct virtio_pci_notify_cap notify = {
             .cap.cap_len = sizeof notify,
             .notify_off_multiplier =
-                cpu_to_le32(QEMU_VIRTIO_PCI_QUEUE_MEM_MULT),
+                cpu_to_le32(virtio_pci_queue_mem_mult(proxy)),
         };
         struct virtio_pci_cfg_cap cfg = {
             .cap.cap_len = sizeof cfg,
@@ -1744,8 +1751,7 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error 
**errp)
     proxy->device.type = VIRTIO_PCI_CAP_DEVICE_CFG;

     proxy->notify.offset = 0x3000;
-    proxy->notify.size =
-        QEMU_VIRTIO_PCI_QUEUE_MEM_MULT * VIRTIO_QUEUE_MAX;
+    proxy->notify.size = virtio_pci_queue_mem_mult(proxy) * VIRTIO_QUEUE_MAX;
     proxy->notify.type = VIRTIO_PCI_CAP_NOTIFY_CFG;

     proxy->notify_pio.offset = 0x0;
@@ -1754,8 +1760,8 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error 
**errp)

     /* subclasses can enforce modern, so do this unconditionally */
     memory_region_init(&proxy->modern_bar, OBJECT(proxy), "virtio-pci",
-                       2 * QEMU_VIRTIO_PCI_QUEUE_MEM_MULT *
-                       VIRTIO_QUEUE_MAX);
+                       /* PCI BAR regions must be powers of 2 */
+                       pow2ceil(proxy->notify.offset + proxy->notify.size));

     memory_region_init_alias(&proxy->modern_cfg,
                              OBJECT(proxy),
@@ -1833,6 +1839,8 @@ static Property virtio_pci_properties[] = {
                     VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT, false),
     DEFINE_PROP_BIT("x-disable-pcie", VirtIOPCIProxy, flags,
                     VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT, false),
+    DEFINE_PROP_BIT("page-per-vq", VirtIOPCIProxy, flags,
+                    VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT, false),
     DEFINE_PROP_END_OF_LIST(),
 };

diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
index 25fbf8a..a745512 100644
--- a/hw/virtio/virtio-pci.h
+++ b/hw/virtio/virtio-pci.h
@@ -64,6 +64,7 @@ enum {
     VIRTIO_PCI_FLAG_MIGRATE_EXTRA_BIT,
     VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT,
     VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT,
+    VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT,
 };

 /* Need to activate work-arounds for buggy guests at vmstate load. */
@@ -84,6 +85,10 @@ enum {
 #define VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY \
     (1 << VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT)

+/* page per vq flag to be used by split drivers within guests */
+#define VIRTIO_PCI_FLAG_PAGE_PER_VQ \
+    (1 << VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT)
+
 typedef struct {
     MSIMessage msg;
     int virq;
diff --git a/include/hw/compat.h b/include/hw/compat.h
index 08dd4fb..a1d6694 100644
--- a/include/hw/compat.h
+++ b/include/hw/compat.h
@@ -2,7 +2,11 @@
 #define HW_COMPAT_H

 #define HW_COMPAT_2_7 \
-        /* empty */
+    {\
+        .driver   = "virtio-pci",\
+        .property = "page-per-vq",\
+        .value    = "on",\
+    },

 #define HW_COMPAT_2_6 \
     {\
--
2.5.5
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