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 <dvlas...@redhat.com>
> CC: Linus Torvalds <torva...@linux-foundation.org>
> CC: Steven Rostedt <rost...@goodmis.org>
> CC: Ingo Molnar <mi...@kernel.org>
> CC: Borislav Petkov <b...@alien8.de>
> CC: "H. Peter Anvin" <h...@zytor.com>
> CC: Andy Lutomirski <l...@amacapital.net>
> CC: Oleg Nesterov <o...@redhat.com>
> CC: Frederic Weisbecker <fweis...@gmail.com>
> CC: Alexei Starovoitov <a...@plumgrid.com>
> CC: Will Drewry <w...@chromium.org>
> CC: Kees Cook <keesc...@chromium.org>
> CC: x...@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 <b...@suse.de>
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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