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From: Denys Vlasenko <vda.linux@googlemail.com>
To: Andy Lutomirski <luto@amacapital.net>
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 15:00:26 +0100 [thread overview]
Message-ID: <CAK1hOcOjNfyrb3k5euNHDQdc-2kKKi2DkBev+LLkcVd4VUszyQ@mail.gmail.com> (raw)
In-Reply-To: <CALCETrW6-ToMwhfSzA79K2zz6iPNoz0EjPa_wHK7jiJLip8zag@mail.gmail.com>
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.
next prev parent reply other threads:[~2015-03-10 14:00 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 [this message]
2015-03-10 14:02 ` Andy Lutomirski
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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