[Qemu-devel] [PATCH v3 1/1] hyperv: cpu hotplug fix with HyperV enabled

["Denis V. Lunev" <den@openvz.org>]

With Hyper-V enabled CPU hotplug stops working. The CPU appears in device
manager on Windows but does not appear in peformance monitor and control
panel.

The root of the problem is the following. Windows checks
HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE bit in CPUID. The presence of
this bit is enough to cure the situation.

The bit should be set when CPU hotplug is allowed for HyperV VM. The check
that hot_add_cpu callback is defined is enough from the protocol point
of view. Though this callback is defined almost always thus there is no
need to export that knowledge in the other way.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Eduardo Habkost <ehabkost@redhat.com>
CC: "Andreas Färber" <afaerber@suse.de>
---
Changes from v2:
- bit set unconditionally upon the discussion

Changes from v1:
- dropped command line option and set the bit if HyperV is enabled and
  hot_add_cpu callback is present

 target-i386/kvm.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/target-i386/kvm.c b/target-i386/kvm.c
index 7974acb..08d6444 100644
--- a/target-i386/kvm.c
+++ b/target-i386/kvm.c
@@ -639,6 +639,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
         if (cpu->hyperv_crash && has_msr_hv_crash) {
             c->edx |= HV_X64_GUEST_CRASH_MSR_AVAILABLE;
         }
+        c->edx |= HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE;
         if (cpu->hyperv_reset && has_msr_hv_reset) {
             c->eax |= HV_X64_MSR_RESET_AVAILABLE;
         }
-- 
2.1.4