linux-kernel - Re: [PATCH v2 4/7] x86/kvm/hyper-v: Introduce KVM_GET_SUPPORTED_HV

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Message-ID: <87bm5sp501.fsf@vitty.brq.redhat.com>
Date:   Tue, 11 Dec 2018 14:28:14 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Roman Kagan <rkagan@...tuozzo.com>
Cc:     "kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "x86\@kernel.org" <x86@...nel.org>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Eduardo Habkost <ehabkost@...hat.com>
Subject: Re: [PATCH v2 4/7] x86/kvm/hyper-v: Introduce KVM_GET_SUPPORTED_HV_CPUID

Roman Kagan <rkagan@...tuozzo.com> writes:

> On Mon, Dec 10, 2018 at 06:21:56PM +0100, Vitaly Kuznetsov wrote:

>> +
>> +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 intended
> usage?

I added HYPERV_CPUID_NESTED_FEATURES in the list (and made the new ioctl
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 can
differ across vCPUs!) with KVM_GET_SUPPORTED_HV_CPUID. We will also have
direct tlb flush and other nested features there so to avoid addning new
KVM_CAP_* for them we need the CPUID.

Another thing I'm thinking about is something like 'hv_all' cpu flag for
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 not
filled when eVMCS was not enabled).

-- 
Vitaly