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

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Date:   Thu, 24 Nov 2022 09:36:24 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Vipin Sharma <vipinsh@...gle.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

Vipin Sharma <vipinsh@...gle.com> writes:

> 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.

The problem is that we don't currently pass 'fast' flag to userspace,
let alone XMM registers. This means that it won't be able to handle fast
hypercalls anyway, I guess it's better to keep your check but add a
comment saying that it's an implementation shortcoming and not a TLFS
requirement.


>
>> > +                     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
>>
>

-- 
Vitaly