linux-kernel - Re: [PATCH] x86: KVM: Add feature flag for AMD's FsGsKernelGsBaseNonSerializing

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Date:   Tue, 3 Oct 2023 21:24:12 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jiaxi Chen <jiaxi.chen@...ux.intel.com>,
        Kim Phillips <kim.phillips@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: KVM: Add feature flag for AMD's FsGsKernelGsBaseNonSerializing

On Tue, Oct 3, 2023 at 8:27 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 10/3/23 19:44, Jim Mattson wrote:
> > I'm a little surprised at the pushback, TBH. Are you implying that
> > there is some advantage to *not* passing this bit through?
>
> I'm not really trying to push back.  I'm honestly just curious.  Linux
> obviously doesn't cat about the bit.  So is this for some future Linux
> or some other OS?

It's not for any particular guest OS. It's just for correctness of the
virtual CPU. Pedantically, hardware that enumerates this bit cannot
run a guest that doesn't. Pragmatically, it almost certainly doesn't
matter. Getting it right is trivial and has no impact on performance
or code size, so why not just do it?