lkml.org 
[lkml]   [2015]   [Mar]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 3/5] x86/entry_64.S: use PUSH insns to build pt_regs on stack
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.
--


\
 
 \ /
  Last update: 2015-03-20 17:41    [W:0.317 / U:0.204 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site