granted devel_ack+ for 3.3 - there are number of bugs we are planning to fic in 3,3 in qxl driver. It will require whql, adding this optimization at the right time.

Proposed patches on mailing list, tested with xp, and with the following system tap script and python script above it (see below after patches).

commit 8c2457c497066b4ff72e32c025faf69fd77b3605
Author: Alon Levy <alevy>
Date:   Mon Jul 29 16:10:09 2013 +0300

    miniport: halve QXL_IO_UPDATE_IRQ calls
    
    RHBZ: 856246

diff --git a/miniport/qxl.c b/miniport/qxl.c
index f5d6b48..629fbdb 100644
--- a/miniport/qxl.c
+++ b/miniport/qxl.c
@@ -1298,7 +1298,7 @@ err:
 VOID InterruptCallback(PVOID dev_extension, PVOID Context)
 {
     QXLExtension *dev_ext = dev_extension;
-    UINT32 pending = VideoPortInterlockedExchange(&dev_ext->ram_header->int_pending, 0);
+    UINT32 pending = (UINT32)Context;
 
     if (pending & QXL_INTERRUPT_DISPLAY) {
         VideoPortSetEvent(dev_ext, dev_ext->display_event);
@@ -1316,14 +1316,14 @@ VOID InterruptCallback(PVOID dev_extension, PVOID Context)
 BOOLEAN Interrupt(PVOID dev_extension)
 {
     QXLExtension *dev_ext = dev_extension;
+    UINT32 pending;
 
-    if (!(dev_ext->ram_header->int_pending & dev_ext->ram_header->int_mask)) {
+    pending = VideoPortInterlockedExchange(&dev_ext->ram_header->int_pending, 0);
+    if (!(pending & dev_ext->ram_header->int_mask)) {
         return FALSE;
     }
-    dev_ext->ram_header->int_mask = 0;
-    VideoPortWritePortUchar((PUCHAR)dev_ext->io_base + QXL_IO_UPDATE_IRQ, 0);
 
-    if (!VideoPortQueueDpc(dev_extension, InterruptCallback, NULL)) {
+    if (!VideoPortQueueDpc(dev_extension, InterruptCallback, (void *)pending)) {
         VideoPortLogError(dev_extension, NULL, E_UNEXPECTED, QXLERR_INT_DELIVERY);
         dev_ext->ram_header->int_mask = WIN_QXL_INT_MASK;
         VideoPortWritePortUchar((PUCHAR)dev_ext->io_base + QXL_IO_UPDATE_IRQ, 0);


--------------------------

To verify number of io exits QXL_IO_UPDATE_IRQ:

U x86_64 garlic:scripts alon (bz917860.v5)$ cat stap-qxl-interrupts
#!/bin/bash -x
STP=$0.stp
ROOT=`dirname $0`/../
sudo stap -v -I $ROOT/x86_64-softmmu $STP

U x86_64 garlic:scripts alon (bz917860.v5)$ cat stap-qxl-interrupts.stp
probe begin { printf("starting qxl generic probe\n") }

probe process("/home/alon/src/spice_rhel6/qemu-kvm-rhel6/x86_64-softmmu/qemu-system-x86_64").statement("*@hw/qxl.c:1396"){
    if ($io_port == 3 /* QXL_IO_UPDATE_IRQ */ ) {
        printf("%d: %s: %s\n", tid(), pp(), $$vars)
    }
}
U x86_64 garlic:scripts alon (bz917860.v5)$ cat watch-interrupts 
#!/usr/bin/python

import subprocess
import time
import select

p = subprocess.Popen(args=['./stap-qxl-interrupts'], stdout=subprocess.PIPE)

while True:
    start = time.time()
    rlist, wlist, xlist = select.select([p.stdout.fileno()], [], [], 1)
    if len(rlist) != 0:
        data = []
        now = time.time()
        while len(rlist) > 0 and now - start < 1:
            data.append(p.stdout.readline())
            rlist, wlist, xlist = select.select([p.stdout.fileno()], [], [], 1)
            now = time.time()
        print "%d" % len(data)
    else:
        print "nothing"
    now = time.time()
    if now < start + 1:
        time.sleep(start + 1 - now)

moving to 3.4 as we are at RC phase, and we don't a solution yet.

no plans for whql in 3.4, moving to the future releases.

"solution" was buggy and reverted, setting back to assigned and lowering priority.

oVirt 4.0 Alpha has been released, moving to oVirt 4.0 Beta target.