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Subject: Re: [PATCH v2] KVM: X86: deprecate obsolete KVM_GET_CPUID2 ioctl
To:     Jim Mattson <jmattson@google.com>
Cc:     linmiaohe <linmiaohe@huawei.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
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        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <e173c489-dee7-a86d-3ec4-6fe45938a2d8@redhat.com>
Date:   Mon, 2 Mar 2020 19:02:39 +0100
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On 02/03/20 18:44, Jim Mattson wrote:
> On Mon, Mar 2, 2020 at 9:09 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> On 02/03/20 18:01, Jim Mattson wrote:
>>>> And in fact, it's not used anywhere. So it should be
>>>> deprecated.
>>> I don't know how you can make the assertion that this ioctl is not
>>> used anywhere. For instance, I see a use of it in Google's code base.
>>
>> Right, it does not seem to be used anywhere according to e.g. Debian
>> code search but of course it can have users.
>>
>> What are you using it for?  It's true that cpuid->nent is never written
>> back to userspace, so the ioctl is basically unusable unless you already
>> know how many entries are written.  Or unless you fill the CPUID entries
>> with garbage before calling it, I guess; is that what you are doing?
> 
> One could use GET_CPUID2 after SET_CPUID2, to see what changes kvm
> made to the requested guest CPUID information without telling you.

Yeah, I think GET_CPUID2 with the same number of leaves that you have
passed to SET_CPUID2 should work.

Paolo