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.
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