On Fri, Jan 13, 2023 at 12:32:17PM -0800, Daniel Verkamp wrote: > The x86-64 memmove code has two ALTERNATIVE statements in a row, one to > handle FSRM ("Fast Short REP MOVSB"), and one to handle ERMS ("Enhanced > REP MOVSB"). If either of these features is present, the goal is to jump > directly to a REP MOVSB; otherwise, some setup code that handles short > lengths is executed. The first comparison of a sequence of specific > small sizes is included in the first ALTERNATIVE, so it will be replaced > by NOPs if FSRM is set, and then (assuming ERMS is also set) execution > will fall through to the JMP to a REP MOVSB in the next ALTERNATIVE. > > The two ALTERNATIVE invocations can be combined into a single instance > of ALTERNATIVE_2 to simplify and slightly shorten the code. If either > FSRM or ERMS is set, the first instruction in the memmove_begin_forward > path will be replaced with a jump to the REP MOVSB. > > This also prevents a problem when FSRM is set but ERMS is not; in this > case, the previous code would have replaced both ALTERNATIVEs with NOPs > and skipped the first check for sizes less than 0x20 bytes. This > combination of CPU features is arguably a firmware bug, but this patch > makes the function robust against this badness. > > Fixes: f444a5ff95dc ("x86/cpufeatures: Add support for fast short REP; MOVSB") > Signed-off-by: Daniel Verkamp <dverkamp@xxxxxxxxxxxx> > --- > arch/x86/lib/memmove_64.S | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/lib/memmove_64.S b/arch/x86/lib/memmove_64.S > index 724bbf83eb5b..1fc36dbd3bdc 100644 > --- a/arch/x86/lib/memmove_64.S > +++ b/arch/x86/lib/memmove_64.S > @@ -38,8 +38,10 @@ SYM_FUNC_START(__memmove) > > /* FSRM implies ERMS => no length checks, do the copy directly */ > .Lmemmove_begin_forward: > - ALTERNATIVE "cmp $0x20, %rdx; jb 1f", "", X86_FEATURE_FSRM > - ALTERNATIVE "", "jmp .Lmemmove_erms", X86_FEATURE_ERMS > + ALTERNATIVE_2 \ > + "cmp $0x20, %rdx; jb 1f", \ > + "jmp .Lmemmove_erms", X86_FEATURE_FSRM, \ > + "jmp .Lmemmove_erms", X86_FEATURE_ERMS > > /* > * movsq instruction have many startup latency > -- > 2.39.0.314.g84b9a713c41-goog > <formletter> This is not the correct way to submit patches for inclusion in the stable kernel tree. Please read: https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html for how to do this properly. </formletter>