linux-kernel - Re: [PATCH v2] KVM: X86: deprecate obsolete KVM_GET

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Date:   Mon, 2 Mar 2020 09:44:45 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linmiaohe <linmiaohe@...wei.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v2] KVM: X86: deprecate obsolete KVM_GET_CPUID2 ioctl

On Mon, Mar 2, 2020 at 9:09 AM Paolo Bonzini <pbonzini@...hat.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.