On Tue, Oct 24, 2023 at 09:35:15AM -0700, Pawan Gupta wrote: > On Tue, Oct 24, 2023 at 12:36:01PM +0200, Peter Zijlstra wrote: > > On Tue, Oct 24, 2023 at 01:08:21AM -0700, Pawan Gupta wrote: > > > > > +.macro CLEAR_CPU_BUFFERS > > > + ALTERNATIVE "jmp .Lskip_verw_\@;", "jmp .Ldo_verw_\@", X86_FEATURE_CLEAR_CPU_BUF > > > + /* nopl __KERNEL_DS(%rax) */ > > > + .byte 0x0f, 0x1f, 0x80, 0x00, 0x00; > > > +.Lverw_arg_\@: .word __KERNEL_DS; > > > +.Ldo_verw_\@: verw _ASM_RIP(.Lverw_arg_\@); > > > +.Lskip_verw_\@: > > > +.endm > > > > Why can't this be: > > > > ALTERNATIVE "". "verw _ASM_RIP(mds_verw_sel)", X86_FEATURE_CLEAR_CPU_BUF > > > > And have that mds_verw_sel thing be out-of-line ? > > I haven't done this way because its a tad bit fragile as it depends on > modules being within 4GB of kernel. We 100% rely on that *everywhere*, nothing fragile about it.
