Re: [PATCH v2 3/3] KVM: SVM: Advertise Translation Cache Extensions to userspace

[Yosry Ahmed]

On Fri, Mar 6, 2026 at 5:54 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> > From: Venkatesh Srinivas <venkateshs@xxxxxxxxxxxx>
> >
> > TCE augments the behavior of TLB invalidating instructions (INVLPG,
> > INVLPGB, and INVPCID) to only invalidate translations for relevant
> > intermediate mappings to the address range, rather than ALL intermdiate
> > translations.
> >
> > The Linux kernel has been setting EFER.TCE if supported by the CPU since
> > commit 440a65b7d25f ("x86/mm: Enable AMD translation cache extensions"),
> > as it may improve performance.
> >
> > KVM does not need to do anything to virtualize the feature, only
> > advertise it and allow setting EFER.TCE. If a TLB invalidating
> > instruction is not intercepted, it will behave according to the guest's
> > setting of EFER.TCE as the value will be loaded on VM-Enter. Otherwise,
> > KVM's emulation may invalidate more TLB entries, which is perfectly fine
> > as the CPU is allowed to invalidate more TLB entries that it strictly
> > needs to.
> >
> > Advertise X86_FEATURE_TCE to userspace, and allow the guest to set
> > EFER.TCE if available.
> >
> > Signed-off-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxxxx>
> > Co-developed-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> > Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
>
> I'll repeat what I said on that referenced patch.
>
> What's the point?  AMD have said that TCE doesn't exist any more; it's a
> bit that's no longer wired into anything.
>
> You've got to get to pre-Zen hardware before this has any behavioural
> effect, at which point the breath of testing is almost 0.

Oh, I did not know that, thanks for pointing it out.

I'll leave it up to Sean whether to pick this up (because Linux guests
still set the bit), just pick up patches 1-2 as cleanups, or drop this
entirely.