linux-kernel - Re: [PATCH 0/7] KVM: x86: hyper-v: make KVM_GET_SUPPORTED_HV

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Message-ID: <87blipwnpn.fsf@vitty.brq.redhat.com>
Date:   Tue, 01 Sep 2020 16:55:48 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        Jon Doron <arilou@...il.com>, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 0/7] KVM: x86: hyper-v: make KVM_GET_SUPPORTED_HV_CPUID more useful

Vitaly Kuznetsov <vkuznets@...hat.com> writes:

> KVM_GET_SUPPORTED_HV_CPUID was initially implemented as a vCPU ioctl but
> this is not very useful when VMM is just trying to query which Hyper-V
> features are supported by the host prior to creating VM/vCPUs. The data
> in KVM_GET_SUPPORTED_HV_CPUID is mostly static with a few exceptions but
> it seems we can change this. Add support for KVM_GET_SUPPORTED_HV_CPUID as
> a system ioctl as well.
>
> QEMU specific description:
> In some cases QEMU needs to collect the information about which Hyper-V
> features are supported by KVM and pass it up the stack. For non-hyper-v
> features this is done with system-wide KVM_GET_SUPPORTED_CPUID/
> KVM_GET_MSRS ioctls but Hyper-V specific features don't get in the output
> (as Hyper-V CPUIDs intersect with KVM's). In QEMU, CPU feature expansion
> happens before any KVM vcpus are created so KVM_GET_SUPPORTED_HV_CPUID
> can't be used in its current shape.
>

Ping ;-)

I know Hyper-V emulation in KVM is a very specific topic so it's not
very easy to find reviewers but I'd be very happy if someone could
provide any feedback on the idea (above) in general.

-- 
Vitaly