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From:   Roman Kagan <rkagan@virtuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
CC:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?iso-8859-2?Q?Radim_Kr=E8m=E1=F8?= <rkrcmar@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "x86@kernel.org" <x86@kernel.org>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com>,
        Eduardo Habkost <ehabkost@redhat.com>
Subject: Re: [PATCH v2 4/7] x86/kvm/hyper-v: Introduce
 KVM_GET_SUPPORTED_HV_CPUID
Thread-Topic: [PATCH v2 4/7] x86/kvm/hyper-v: Introduce
 KVM_GET_SUPPORTED_HV_CPUID
Thread-Index: AQHUkKznsqV3L84bk0eamd/uIlqfEKV5sPkA///YaQCAADvygP//3tGAgAABsQA=
Date:   Tue, 11 Dec 2018 15:10:08 +0000
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On Tue, Dec 11, 2018 at 04:04:01PM +0100, Vitaly Kuznetsov wrote:
> Roman Kagan <rkagan@virtuozzo.com> writes:
>=20
> > On Tue, Dec 11, 2018 at 02:28:14PM +0100, Vitaly Kuznetsov wrote:
> >> Roman Kagan <rkagan@virtuozzo.com> writes:
> >>=20
> >> > On Mon, Dec 10, 2018 at 06:21:56PM +0100, Vitaly Kuznetsov wrote:
> >>=20
> >> >> +
> >> >> +Currently, the following list of CPUID leaves are returned:
> >> >> + HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS
> >> >> + HYPERV_CPUID_INTERFACE
> >> >> + HYPERV_CPUID_VERSION
> >> >> + HYPERV_CPUID_FEATURES
> >> >> + HYPERV_CPUID_ENLIGHTMENT_INFO
> >> >> + HYPERV_CPUID_IMPLEMENT_LIMITS
> >> >> + HYPERV_CPUID_NESTED_FEATURES
> >> >> +
> >> >> +HYPERV_CPUID_NESTED_FEATURES leaf is only exposed when Enlightened=
 VMCS was
> >> >> +enabled on the corresponding vCPU (KVM_CAP_HYPERV_ENLIGHTENED_VMCS=
).
> >> >
> >> > IOW the output of ioctl(KVM_GET_SUPPORTED_HV_CPUID) depends on
> >> > whether ioctl(KVM_ENABLE_CAP, KVM_CAP_HYPERV_ENLIGHTENED_VMCS) has
> >> > already been called on that vcpu?  I wonder if this fits the intende=
d
> >> > usage?
> >>=20
> >> I added HYPERV_CPUID_NESTED_FEATURES in the list (and made the new ioc=
tl
> >> per-cpu and not per-vm) for consistency. *In theory*
> >> KVM_CAP_HYPERV_ENLIGHTENED_VMCS is also enabled per-vcpu so some
> >> hypothetical userspace can later check enabled eVMCS versions (which c=
an
> >> differ across vCPUs!) with KVM_GET_SUPPORTED_HV_CPUID. We will also ha=
ve
> >> direct tlb flush and other nested features there so to avoid addning n=
ew
> >> KVM_CAP_* for them we need the CPUID.
> >
> > This is different from how KVM_GET_SUPPORTED_CPUID is used: QEMU assume=
s
> > that its output doesn't change between calls, and even caches the resul=
t
> > calling the ioctl only once.
> >
>=20
> Yes, I'm not sure if we have to have full consistency between
> KVM_GET_SUPPORTED_CPUID and KVM_GET_SUPPORTED_HV_CPUID.

Neither do I.  I just noticed the difference and thought it might
matter.

> >> Another thing I'm thinking about is something like 'hv_all' cpu flag f=
or
> >> Qemu which would enable everything by setting guest CPUIDs to what
> >> KVM_GET_SUPPORTED_HV_CPUID returns. In that case it would also be
> >> convenient to have HYPERV_CPUID_NESTED_FEATURES properly filled (or no=
t
> >> filled when eVMCS was not enabled).
> >
> > I think this is orthogonal to the way you obtain capability info from
> > the kernel.
>=20
> Not necessarily. If very dumb userspace does 'host passthrough' for
> Hyper-V features without doing anything (e.g. not enabling Enlightened
> VMCS) it will just put the result of KVM_GET_SUPPORTED_HV_CPUID in guest
> facing CPUIDs and it will all work. In case eVMCS was previously enabled
> it again just copies everything and this still works.
>=20
> We don't probably need this for Qemu though. If you think it would be
> better to have HYPERV_CPUID_NESTED_FEATURES returned regardless of eVMCS
> enablement I'm ready to budge)

I have no opinion on this.  I hope Paolo, who requested the feature, can
explain the desired semantics :)

Roman.