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    SubjectRe: [PATCH v2 4/7] KVM: x86: hyper-v: always advertise HV_STIMER_DIRECT_MODE_AVAILABLE
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    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@redhat.com>
    > ---
    > 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

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