linux-kernel - Re: [PATCH 3/5] x86/entry_64.S: use PUSH insns to build pt

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Date:	Fri, 20 Mar 2015 17:35:29 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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@...hat.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Ingo Molnar <mingo@...nel.org>
> CC: Borislav Petkov <bp@...en8.de>
> CC: "H. Peter Anvin" <hpa@...or.com>
> CC: Andy Lutomirski <luto@...capital.net>
> CC: Oleg Nesterov <oleg@...hat.com>
> CC: Frederic Weisbecker <fweisbec@...il.com>
> CC: Alexei Starovoitov <ast@...mgrid.com>
> CC: Will Drewry <wad@...omium.org>
> CC: Kees Cook <keescook@...omium.org>
> CC: x86@...nel.org
> CC: linux-kernel@...r.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@...e.de>

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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