linux-kernel - Re: [PATCH 3/4] x86: save user rsp in pt

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Date:	Tue, 10 Mar 2015 15:00:26 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"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 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86: save user rsp in pt_regs->sp on SYSCALL64 fastpath

On Tue, Mar 10, 2015 at 2:26 PM, Andy Lutomirski <luto@...capital.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.
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