Re: [PATCH v7 8/9] i386: Hyper-V SynIC requires POST_MESSAGES/SIGNAL_EVENTS priviliges

[Eduardo Habkost <ehabkost@redhat.com>]

On Thu, Jun 03, 2021 at 01:48:34PM +0200, Vitaly Kuznetsov wrote:
> When Hyper-V SynIC is enabled, we may need to allow Windows guests to make
> hypercalls (POST_MESSAGES/SIGNAL_EVENTS). No issue is currently observed
> because KVM is very permissive, allowing these hypercalls regarding of
> guest visible CPUid bits.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  target/i386/kvm/hyperv-proto.h | 6 ++++++
>  target/i386/kvm/kvm.c          | 6 ++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/target/i386/kvm/hyperv-proto.h b/target/i386/kvm/hyperv-proto.h
> index e30d64b4ade4..5fbb385cc136 100644
> --- a/target/i386/kvm/hyperv-proto.h
> +++ b/target/i386/kvm/hyperv-proto.h
> @@ -38,6 +38,12 @@
>  #define HV_ACCESS_FREQUENCY_MSRS     (1u << 11)
>  #define HV_ACCESS_REENLIGHTENMENTS_CONTROL  (1u << 13)
>  
> +/*
> + * HV_CPUID_FEATURES.EBX bits
> + */
> +#define HV_POST_MESSAGES             (1u << 4)
> +#define HV_SIGNAL_EVENTS             (1u << 5)
> +
>  /*
>   * HV_CPUID_FEATURES.EDX bits
>   */
> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> index a3897d4d8788..6a32d43e6ec1 100644
> --- a/target/i386/kvm/kvm.c
> +++ b/target/i386/kvm/kvm.c
> @@ -1343,6 +1343,12 @@ static int hyperv_fill_cpuids(CPUState *cs,
>      /* Unconditionally required with any Hyper-V enlightenment */
>      c->eax |= HV_HYPERCALL_AVAILABLE;
>  
> +    /* SynIC and Vmbus devices require messages/signals hypercalls */
> +    if (hyperv_feat_enabled(cpu, HYPERV_FEAT_SYNIC) &&
> +        !cpu->hyperv_synic_kvm_only) {
> +        c->ebx |= HV_POST_MESSAGES | HV_SIGNAL_EVENTS;

Why exactly is the hyperv_synic_kvm_only check needed?

Is the hyperv_synic_kvm_only check the only reason this was done
here and not at kvm_hyperv_properties?


> +    }
> +
>      /* Not exposed by KVM but needed to make CPU hotplug in Windows work */
>      c->edx |= HV_CPU_DYNAMIC_PARTITIONING_AVAILABLE;
>  
> -- 
> 2.31.1
> 

-- 
Eduardo