Re: [tip: x86/urgent] x86/bugs: Add asm helpers for executing VERW

[Pawan Gupta]

On Mon, Feb 26, 2024 at 09:17:30AM +0200, Nikolay Borisov wrote:
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index 262e655..077083e 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -315,6 +315,17 @@
> >   #endif
> >   .endm
> > +/*
> > + * Macro to execute VERW instruction that mitigate transient data sampling
> > + * attacks such as MDS. On affected systems a microcode update overloaded VERW
> > + * instruction to also clear the CPU buffers. VERW clobbers CFLAGS.ZF.
> > + *
> > + * Note: Only the memory operand variant of VERW clears the CPU buffers.
> > + */
> > +.macro CLEAR_CPU_BUFFERS
> > +	ALTERNATIVE "", __stringify(verw _ASM_RIP(mds_verw_sel)), X86_FEATURE_CLEAR_CPU_BUF
> 
> Any particular reason why this uses RIP-relative vs an absolute address
> mode?

Early versions of the series had the VERW arg pointing to the macro
itself, that is why relative addressing was used. That got changed in a
later version with all VERW sites pointing to a single memory location.

> I know in our private exchange you said there is no significance but
> for example older kernels have a missing relocation support in alternatives.
> This of course can be worked around by slightly changing the logic of the
> macro which means different kernels will have slightly different macros.

Do you anticipate a problem with that? If yes, I can send a patch to use
fixed addressing in upstream as well.