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From: Andy Lutomirski <luto@amacapital.net>
To: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Ingo Molnar <mingo@kernel.org>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Alexei Starovoitov <ast@plumgrid.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	X86 ML <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath
Date: Tue, 10 Mar 2015 07:02:08 -0700	[thread overview]
Message-ID: <CALCETrV=n2AYVObh0URscYbRb2o+wBS9KSYOq==V92UXRmpj0Q@mail.gmail.com> (raw)
In-Reply-To: <CAK1hOcOjNfyrb3k5euNHDQdc-2kKKi2DkBev+LLkcVd4VUszyQ@mail.gmail.com>

On Tue, Mar 10, 2015 at 7:00 AM, Denys Vlasenko
<vda.linux@googlemail.com> wrote:
> On Tue, Mar 10, 2015 at 2:26 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> usersp is IMO tolerable.  The nasty thing is the FIXUP_TOP_OF_STACK /
>> RESTORE_TOP_OF_STACK garbage, and this patch is the main step toward
>> killing that off completely.  I've still never convinced myself that
>> there aren't ptrace-related info leaks in there.
>>
>> Denys, did you ever benchmark what happens if we use push instead of
>> mov?  I bet that we get that cycle back and more, not to mention much
>> less icache usage.
>
> Yes, I did.
> Push conversion seems to perform the same as current, MOV-based code.
>
> The expected win there that we lose two huge 12-byte insns
> which store __USER_CS and __USER_DS in iret frame.
>
> MOVQ imm,ofs(%rsp) has a very unfortunate encoding in x86:
> - needs REX prefix
> - no sing-extending imm8 form exists for it
> - ofs in our case can't fit into 8 bits
> - (%esp) requires SIB byte
>
> In my tests, each such instruction adds one cycle.
>
> Compare this to PUSH imm8, which is 2 bytes only.

Does that mean that using push on top of this patch gets us our cycle back?

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC

  reply	other threads:[~2015-03-10 14:02 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-09 18:39 [PATCH 0/4 v2] x86: entry_64.S: steps towards simpler iret frame handling Denys Vlasenko
2015-03-09 18:39 ` [PATCH 1/4] x86: save r11 into pt_regs->flags on SYSCALL64 fastpath Denys Vlasenko
2015-03-09 20:02   ` Andy Lutomirski
2015-03-16 12:04   ` [tip:x86/asm] x86/asm/entry/64: Save R11 into pt_regs-> flags " tip-bot for Denys Vlasenko
2015-03-09 18:39 ` [PATCH 3/4] x86: save user rsp in pt_regs->sp " Denys Vlasenko
2015-03-09 20:11   ` Andy Lutomirski
2015-03-09 20:32     ` Denys Vlasenko
2015-03-09 20:43       ` Andy Lutomirski
2015-03-10 12:51   ` Ingo Molnar
2015-03-10 13:10     ` Andy Lutomirski
2015-03-10 13:18     ` Denys Vlasenko
2015-03-10 13:20       ` Andy Lutomirski
2015-03-10 13:26         ` Ingo Molnar
2015-03-10 13:21       ` Ingo Molnar
2015-03-10 13:26         ` Andy Lutomirski
2015-03-10 14:00           ` Denys Vlasenko
2015-03-10 14:02             ` Andy Lutomirski [this message]
2015-03-10 14:09               ` Denys Vlasenko
2015-03-10 13:28         ` Ingo Molnar
2015-03-10 13:50         ` Denys Vlasenko
2015-03-16  9:44           ` Ingo Molnar
2015-03-16 12:05   ` [tip:x86/asm] x86/asm/entry/64: Save user RSP in pt_regs-> sp " tip-bot for Denys Vlasenko
2015-03-10  6:00 ` [PATCH 0/4 v2] x86: entry_64.S: steps towards simpler iret frame handling Ingo Molnar

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