linux-kernel - Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check

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Message-ID: <878rv2izjp.fsf@redhat.com>
Date:   Wed, 26 Jan 2022 11:03:54 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Igor Mammedov <imammedo@...hat.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 2/2] KVM: x86: Use memcmp in kvm_cpuid_check_equal()

Joe Perches <joe@...ches.com> writes:

> On Mon, 2022-01-24 at 11:36 +0100, Vitaly Kuznetsov wrote:
>> kvm_cpuid_check_equal() should also check .flags equality but instead
>> of adding it to the existing check, just switch to using memcmp() for
>> the whole 'struct kvm_cpuid_entry2'.
>
> Is the struct padding guaranteed to be identical ?
>

Well, yes (or we're all doomeed):
- 'struct kvm_cpuid_entry2' is part of KVM userspace ABI, it is supposed
to be stable.
- Here we compare structs which come from the same userspace during one
session (vCPU fd stays open), I can't imagine how structure layout can
change on-the-fly.

-- 
Vitaly