On 26/10/2023 2:44 pm, Nikolay Borisov wrote: > > > <snip> >> + >> +.pushsection .entry.text, "ax" >> + >> +.align L1_CACHE_BYTES, 0xcc >> +SYM_CODE_START_NOALIGN(mds_verw_sel) >> + UNWIND_HINT_UNDEFINED >> + ANNOTATE_NOENDBR >> + .word __KERNEL_DS >> +SYM_CODE_END(mds_verw_sel); >> +/* For KVM */ >> +EXPORT_SYMBOL_GPL(mds_verw_sel); >> + >> +.popsection > > <snip> > >> diff --git a/arch/x86/include/asm/nospec-branch.h >> b/arch/x86/include/asm/nospec-branch.h >> index c55cc243592e..005e69f93115 100644 >> --- a/arch/x86/include/asm/nospec-branch.h >> +++ b/arch/x86/include/asm/nospec-branch.h >> @@ -329,6 +329,21 @@ >> #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. >> + */ >> +.macro EXEC_VERW >> + verw _ASM_RIP(mds_verw_sel) >> +.endm >> + >> +.macro CLEAR_CPU_BUFFERS >> + ALTERNATIVE "", __stringify(EXEC_VERW), X86_FEATURE_CLEAR_CPU_BUF >> +.endm > > > What happened with the first 5 bytes of a 7 byte nop being > complemented by __KERNEL_DS in order to handle VERW being executed > after user registers are restored and having its memory operand ? It was moved out of line (so no need to hide a constant in a nop), deduped, and renamed to mds_verw_sel. verw _ASM_RIP(mds_verw_sel) *is* the memory form. ~Andrew
