Re: [PATCH v2 4/7] KVM: x86: hyper-v: always advertise HV_STIMER_DIRECT_MODE_AVAILABLE

[Paolo Bonzini]

On 24/09/20 16:57, Vitaly Kuznetsov wrote:
> HV_STIMER_DIRECT_MODE_AVAILABLE is the last conditionally set feature bit
> in KVM_GET_SUPPORTED_HV_CPUID but it doesn't have to be conditional: first,
> this bit is only an indication to userspace VMM that direct mode stimers
> are supported, it still requires manual enablement (enabling SynIC) to
> work so no VMM should just blindly copy it to guest CPUIDs. Second,
> lapic_in_kernel() is a must for SynIC. Expose the bit unconditionally.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/kvm/hyperv.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 6da20f91cd59..503829f71270 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -2028,13 +2028,7 @@ int kvm_vcpu_ioctl_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
>  			ent->ebx |= HV_DEBUGGING;
>  			ent->edx |= HV_X64_GUEST_DEBUGGING_AVAILABLE;
>  			ent->edx |= HV_FEATURE_DEBUG_MSRS_AVAILABLE;
> -
> -			/*
> -			 * Direct Synthetic timers only make sense with in-kernel
> -			 * LAPIC
> -			 */
> -			if (lapic_in_kernel(vcpu))
> -				ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;
> +			ent->edx |= HV_STIMER_DIRECT_MODE_AVAILABLE;
>  
>  			break;
>  
> 

Sorry for the late reply.  I think this is making things worse.  It's
obviously okay to add a system KVM_GET_SUPPORTED_HV_CPUID, and I guess
it makes sense to have bits in there that require to enable a
capability.  For example, KVM_GET_SUPPORTED_CPUID has a couple bits such
as X2APIC, that we return even if they require in-kernel irqchip.

For the vCPU version however we should be able to copy the returned
leaves to KVM_SET_CPUID2, meaning that unsupported features should be
masked.

Paolo