On Tue, Oct 24, 2023 at 09:00:29PM -0700, Pawan Gupta wrote: > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h > index c55cc243592e..ed8218e2d9a7 100644 > --- a/arch/x86/include/asm/nospec-branch.h > +++ b/arch/x86/include/asm/nospec-branch.h > @@ -13,6 +13,7 @@ > #include <asm/unwind_hints.h> > #include <asm/percpu.h> > #include <asm/current.h> > +#include <asm/segment.h> > > /* > * Call depth tracking for Intel SKL CPUs to address the RSB underflow > @@ -329,6 +330,29 @@ > #endif > .endm > > +/* > + * Macros 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. > + */ > +.pushsection .rodata > +.align 64 > +mds_verw_sel: > + .word __KERNEL_DS > + .byte 0xcc > +.align 64 > +.popsection This should not be in a header file, you'll get an instance of this per translation unit, not what you want. > + > +.macro EXEC_VERW > + verw _ASM_RIP(mds_verw_sel) > +.endm > + > +.macro CLEAR_CPU_BUFFERS > + ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF > +.endm > + > #else /* __ASSEMBLY__ */
