linux-kernel - Re: [PATCH 1/2] KVM: x86: Advertise CPUID.(EAX=7,ECX=2):EDX[5:0] to userspace

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Date:   Thu, 30 Nov 2023 20:18:19 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 1/2] KVM: x86: Advertise CPUID.(EAX=7,ECX=2):EDX[5:0] to userspace

On Thu, Nov 30, 2023 at 5:54 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Mon, 23 Oct 2023 17:16:35 -0700, Jim Mattson wrote:
> > The low five bits {INTEL_PSFD, IPRED_CTRL, RRSBA_CTRL, DDPD_U, BHI_CTRL}
> > advertise the availability of specific bits in IA32_SPEC_CTRL. Since KVM
> > dynamically determines the legal IA32_SPEC_CTRL bits for the underlying
> > hardware, the hard work has already been done. Just let userspace know
> > that a guest can use these IA32_SPEC_CTRL bits.
> >
> > The sixth bit (MCDT_NO) states that the processor does not exhibit MXCSR
> > Configuration Dependent Timing (MCDT) behavior. This is an inherent
> > property of the physical processor that is inherited by the virtual
> > CPU. Pass that information on to userspace.
> >
> > [...]
>
> Applied to kvm-x86 misc, with macros to make Jim queasy (but they really do
> guard against copy+paste errors).

They are also quite effective at guarding against code search. :)

> [1/2] KVM: x86: Advertise CPUID.(EAX=7,ECX=2):EDX[5:0] to userspace
>       https://github.com/kvm-x86/linux/commit/eefe5e668209
> [2/2] KVM: x86: Use a switch statement and macros in __feature_translate()
>       https://github.com/kvm-x86/linux/commit/80c883db87d9
>
> --
> https://github.com/kvm-x86/linux/tree/next