Messages in this thread | | | Date | Fri, 20 Mar 2015 17:35:29 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH 3/5] x86/entry_64.S: use PUSH insns to build pt_regs on stack |
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On Thu, Mar 19, 2015 at 06:17:47PM +0100, Denys Vlasenko wrote: > With this change, on SYSCALL64 code path we are now populating > pt_regs->cs, pt_regs->ss and pt_regs->rcx unconditionally and > therefore don't need to do that in FIXUP_TOP_OF_STACK. > > We lose a number of large insns there: > > text data bss dec hex filename > 13298 0 0 13298 33f2 entry_64_before.o > 12978 0 0 12978 32b2 entry_64.o > > What's more important, we convert two "MOVQ $imm,off(%rsp)" to "PUSH $imm" > (the ones which fill pt_regs->cs,ss). > > Before this patch, placing them on fast path was slowing it down by two cycles: > this form of MOV is very large, 12 bytes, and this probably reduces decode bandwidth > to one insn per cycle when CPU sees them. > > Therefore they were living in FIXUP_TOP_OF_STACK instead (away from fast path). > > "PUSH $imm" is a small 2-byte insn. Moving it to fast path does not slow it down > in my measurements. > > Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com> > CC: Linus Torvalds <torvalds@linux-foundation.org> > CC: Steven Rostedt <rostedt@goodmis.org> > CC: Ingo Molnar <mingo@kernel.org> > CC: Borislav Petkov <bp@alien8.de> > CC: "H. Peter Anvin" <hpa@zytor.com> > CC: Andy Lutomirski <luto@amacapital.net> > CC: Oleg Nesterov <oleg@redhat.com> > CC: Frederic Weisbecker <fweisbec@gmail.com> > CC: Alexei Starovoitov <ast@plumgrid.com> > CC: Will Drewry <wad@chromium.org> > CC: Kees Cook <keescook@chromium.org> > CC: x86@kernel.org > CC: linux-kernel@vger.kernel.org > --- > > Changes since last version: reformulate old comment which was > mostly failing to explain why we don't have TRACE_IRQS_OFF/ONs > around a irq-off section in SYSCALL64 code path. > > arch/x86/kernel/entry_64.S | 54 +++++++++++++++++++++++++++------------------- > 1 file changed, 32 insertions(+), 22 deletions(-)
Acked-by: Borislav Petkov <bp@suse.de>
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply. --
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