Re: Save a WRMSR GS.base?

[Borislav Petkov]

On Thu, Jun 04, 2026 at 09:30:33PM -0700, H. Peter Anvin wrote:
> On June 4, 2026 9:26:52 PM PDT, Borislav Petkov <bp@xxxxxxxxx> wrote:
> >On Thu, Jun 04, 2026 at 08:20:57PM -0700, H. Peter Anvin wrote:
> >> I guess the question is why there is a "first" one.
> >
> >That happens when we do:
> >
> >x86_fsgsbase_load()
> >
> >	loadseg(GS) -> load_gs_index() -> native_load_gs_index() ->
> >	  if (cpu_feature_enabled(X86_FEATURE_LKGS))
> >                native_lkgs(selector);
> >
> >then back in x86_fsgsbase_load() we do:
> >
> >	__wrgsbase_inactive(next->gsbase);
> >
> >which does
> >
> >	wrmsrq(MSR_KERNEL_GS_BASE, gsbase);
> >
> >on FRED.
> >
> >But LKGS already wrote MSR_KERNEL_GS_BASE...
> >
> >> Logically the sequence should be LKGS first, if needed; then WRMSR(NS). LKGS
> >> can be replaced with swapgs/mov gs/swapgs on legacy.
> >
> >Right.
> >
> >I think avoiding that second WRMSR(MSR_KERNEL_GS_BASE) should give some perf
> >back...
> >
> >Although, I need to think how to make it pretty...
> >
> 
> Should be doing wrmsrns...

No, I think that second WRMSR* should not happen at all if we have executed
LKGS which has already written MSR_KERNEL_GS_BASE, right?


-- 
Regards/Gruss,
    Boris.

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