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

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Message-ID: <a1ff3db1-1f5a-7bab-6c4b-f76e6d76d468@redhat.com>
Date:   Mon, 2 Mar 2020 18:09:39 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>, linmiaohe <linmiaohe@...wei.com>
Cc:     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 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?

Paolo