linux-kernel - Re: [PATCH v4] KVM: x86: Fix KVM_GET_CPUID2 ioctl to return cpuid entries count

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Date:   Tue, 4 May 2021 11:50:20 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Alexander Graf <graf@...zon.com>,
        "Denis V. Lunev" <den@...nvz.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Valeriy Vdovin <valeriy.vdovin@...tuozzo.com>
Cc:     linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Shuah Khan <shuah@...nel.org>,
        Aaron Lewis <aaronlewis@...gle.com>,
        Andrew Jones <drjones@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        Like Xu <like.xu@...ux.intel.com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v4] KVM: x86: Fix KVM_GET_CPUID2 ioctl to return cpuid
 entries count

On 04/05/21 11:26, Alexander Graf wrote:
> 
> 
> On 04.05.21 10:21, Paolo Bonzini wrote:
>>
>> On 04/05/21 10:15, Denis V. Lunev wrote:
>>> As far as I understand only some testing within kernel now.
>>> Though we have plans to expose it for QAPI as the series
>>> in QEMU
>>>    [PATCH 1/2] qapi: fix error handling for x-vz-query-cpu-model-cpuid
>>>    [PATCH 2/2] qapi: blacklisted x-vz-query-cpu-model-cpuid in tests
>>> is not coming in a good way.
>>> The idea was to avoid manual code rework in QEMU and
>>> expose collected model at least for debug.
>>
>> KVM_GET_CPUID2 as a VM ioctl cannot expose the whole truth about CPUID
>> either, since it doesn't handle the TSX_CTRL_CPUID_CLEAR bit.  Given
>> that QEMU doesn't need KVM_GET_CPUID2; it only needs to save whatever it
>> passed to KVM_SET_CPUID2.
> 
> What if we instead deflect CPUID into user space so it can emulate it in 
> whatever way it likes? Is the performance difference going to be 
> relevant? Are people still using cpuid as barrier these days?

There's enough weirdness in CPUID (e.g. the magic redirection of unknown 
leaves to the highest Intel leaf) to make it relatively hard to 
implement correctly.  So I think it should remain in the kernel.

Paolo