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

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Date:   Thu, 5 Oct 2023 10:27:42 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...el.com>, 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>,
        Sean Christopherson <seanjc@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86: KVM: Add feature flag for AMD's FsGsKernelGsBaseNonSerializing

On Thu, Oct 5, 2023 at 10:14 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> On 10/5/23 19:06, Jim Mattson wrote:
> > On Thu, Oct 5, 2023 at 9:39 AM Paolo Bonzini<pbonzini@...hat.com>  wrote:
> >
> >> I agree with Jim that it would be nice to have some bits from Intel, and
> >> some bits from AMD, that current processors always return as 1.  Future
> >> processors can change those to 0 as desired.
> > That's not quite what I meant.
> >
> > I'm suggesting a leaf devoted to single bit negative features. If a
> > bit is set in hardware, it means that something has been taken away.
> > Hypervisors don't need to know exactly what was taken away. For this
> > leaf only, hypervisors will always pass through a non-zero bit, even
> > if they have know idea what it means.
>
> Understood, but I'm suggesting that these might even have the right
> polarity: if a bit is set it means that something is there and might not
> in the future, even if we don't know exactly what.  We can pass through
> the bit, we can AND bits across the migration pool to define what to
> pass to the guest, we can force-set the leaves to zero (feature
> removed).  Either way, the point is to group future defeatures together.

Oh, yeah. Your suggestion is better. :)