linux-kernel - Re: [PATCH v2 2/6] KVM: x86: hyper-v: Add extended hypercall support in Hyper-v

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Message-ID: <CAHVum0eW4WMHe1vNsWn-2xbMxgckFwu_pOQR7hs0NbFj3sM8Tg@mail.gmail.com>
Date:   Wed, 23 Nov 2022 11:43:43 -0800
From:   Vipin Sharma <vipinsh@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        seanjc@...gle.com, pbonzini@...hat.com, dmatlack@...gle.com
Subject: Re: [PATCH v2 2/6] KVM: x86: hyper-v: Add extended hypercall support
 in Hyper-v

On Tue, Nov 22, 2022 at 8:29 AM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Vipin Sharma <vipinsh@...gle.com> writes:
>
> > +/*
> > + * The TLFS carves out 64 possible extended hypercalls, numbered sequentially
> > + * after the base capabilities extended hypercall.
> > + */
> > +#define HV_EXT_CALL_MAX (HV_EXT_CALL_QUERY_CAPABILITIES + 64)
> > +
>
> First, I thought there's an off-by-one here (and should be '63') but
> then I checked with TLFS and figured out that the limit comes from
> HvExtCallQueryCapabilities's response which doesn't include itself
> (0x8001) in the mask, this means it can encode
>
> 0x8002 == bit0
> 0x8003 == bit1
> ..
> 0x8041 == bit63
>
> so indeed, the last one supported is 0x8041 == 0x8001 + 64
>
> maybe it's worth extending the commont on where '64' comes from.
>

Yeah, I will expand comments.

> >  static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
> >                               bool vcpu_kick);
> >
> > @@ -2411,6 +2417,9 @@ static bool hv_check_hypercall_access(struct kvm_vcpu_hv *hv_vcpu, u16 code)
> >       case HVCALL_SEND_IPI:
> >               return hv_vcpu->cpuid_cache.enlightenments_eax &
> >                       HV_X64_CLUSTER_IPI_RECOMMENDED;
> > +     case HV_EXT_CALL_QUERY_CAPABILITIES ... HV_EXT_CALL_MAX:
> > +             return hv_vcpu->cpuid_cache.features_ebx &
> > +                             HV_ENABLE_EXTENDED_HYPERCALLS;
> >       default:
> >               break;
> >       }
> > @@ -2564,6 +2573,12 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
> >               }
> >               goto hypercall_userspace_exit;
> >       }
> > +     case HV_EXT_CALL_QUERY_CAPABILITIES ... HV_EXT_CALL_MAX:
> > +             if (unlikely(hc.fast)) {
> > +                     ret = HV_STATUS_INVALID_PARAMETER;
>
> I wasn't able to find any statement in TLFS stating whether extended
> hypercalls can be 'fast', I can imagine e.g. MemoryHeatHintAsync using
> it. Unfortunatelly, our userspace exit will have to be modified to
> handle such stuff. This can stay for the time being I guess..
>

I agree TLFS doesn't state anything about "fast" extended hypercall
but nothing stops in future for some call to be "fast". I think this
condition should also be handled by userspace as it is handling
everything else.

I will remove it in the next version of the patch. I don't see any
value in verification here.

> > +                     break;
> > +             }
> > +             goto hypercall_userspace_exit;
> >       default:
> >               ret = HV_STATUS_INVALID_HYPERCALL_CODE;
> >               break;
> > @@ -2722,6 +2737,7 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid,
> >
> >                       ent->ebx |= HV_POST_MESSAGES;
> >                       ent->ebx |= HV_SIGNAL_EVENTS;
> > +                     ent->ebx |= HV_ENABLE_EXTENDED_HYPERCALLS;
> >
> >                       ent->edx |= HV_X64_HYPERCALL_XMM_INPUT_AVAILABLE;
> >                       ent->edx |= HV_FEATURE_FREQUENCY_MSRS_AVAILABLE;
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>
>
> --
> Vitaly
>