On Wed, Jul 19, 2023 at 12:41:47PM -0700, Sean Christopherson wrote:
> My understanding is that PL[0-2]_SSP are used only on transitions to the
> corresponding privilege level from a *different* privilege level. That means
> KVM should be able to utilize the user_return_msr framework to load the host
> values. Though if Linux ever supports SSS, I'm guessing the core kernel will
> have some sort of mechanism to defer loading MSR_IA32_PL0_SSP until an exit to
> userspace, e.g. to avoid having to write PL0_SSP, which will presumably be
> per-task, on every context switch.
>
> But note my original wording: **If that's necessary**
>
> If nothing in the host ever consumes those MSRs, i.e. if SSS is NOT enabled in
> IA32_S_CET, then running host stuff with guest values should be ok. KVM only
> needs to guarantee that it doesn't leak values between guests. But that should
> Just Work, e.g. KVM should load the new vCPU's values if SHSTK is exposed to the
> guest, and intercept (to inject #GP) if SHSTK is not exposed to the guest.
>
> And regardless of what the mechanism ends up managing SSP MSRs, it should only
> ever touch PL0_SSP, because Linux never runs anything at CPL1 or CPL2, i.e. will
> never consume PL{1,2}_SSP.
To clarify, Linux will only use SSS in FRED mode -- FRED removes CPL1,2.
