Re: [Qemu-devel] [PATCH v2 1/1] hyperv: cpu hotplug fix with HyperV enabled

[Eduardo Habkost]

On Sat, Feb 13, 2016 at 03:00:15PM +0300, Denis V. Lunev wrote:
> 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.

What about live migration? This is going to change CPUID data
under the guest's feet.

> 
> 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.

What would be the consequences of setting it when CPU hotplug is
not available? Is there any real advantage of keeping it unset in
pc-1.4 and older?

If there are good reasons to keep it unset if CPU hotplug is not
possible, why set it when max_cpus == smp_cpus?


> 
> Signed-off-by: Denis V. Lunev <address@hidden>
> Reviewed-by: Roman Kagan <address@hidden>
> CC: Paolo Bonzini <address@hidden>
> CC: Richard Henderson <address@hidden>
> CC: Eduardo Habkost <address@hidden>
> CC: "Andreas Färber" <address@hidden>
> ---
> 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 | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> index 94024bc..63dee9c 100644
> --- a/target-i386/kvm.c
> +++ b/target-i386/kvm.c
> @@ -639,6 +639,9 @@ int kvm_arch_init_vcpu(CPUState *cs)
>          if (cpu->hyperv_crash && has_msr_hv_crash) {
>              c->edx |= HV_X64_GUEST_CRASH_MSR_AVAILABLE;
>          }
> +        if (MACHINE_GET_CLASS(current_machine)->hot_add_cpu != NULL) {
> +            c->edx |= HV_X64_CPU_DYNAMIC_PARTITIONING_AVAILABLE;
> +        }
>          if (cpu->hyperv_reset && has_msr_hv_reset) {
>              c->eax |= HV_X64_MSR_RESET_AVAILABLE;
>          }
> -- 
> 2.5.0
> 
> 

-- 
Eduardo