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

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Date:   Mon, 2 Mar 2020 09:01:27 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     linmiaohe <linmiaohe@...wei.com>
Cc:     Paolo Bonzini <pbonzini@...hat.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 Wed, Feb 26, 2020 at 7:20 PM linmiaohe <linmiaohe@...wei.com> wrote:
>
> From: Miaohe Lin <linmiaohe@...wei.com>
>
> When kvm_vcpu_ioctl_get_cpuid2() fails, we set cpuid->nent to the value of
> vcpu->arch.cpuid_nent. But this is in vain as cpuid->nent is not copied to
> userspace by copy_to_user() from call site. Also cpuid->nent is not updated
> to indicate how many entries were retrieved on success case. So this ioctl
> is straight up broken. 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.