linux-kernel - Re: [PATCH 4/5] x86/asm/entry/32: Replace RESTORE_RSI_RDI[

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Message-ID: <CALCETrUasq4cFacbHhzK-TVBsHD3An3YDSW93kONvaxwnvPv1w@mail.gmail.com>
Date:	Tue, 9 Jun 2015 12:01:03 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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 4/5] x86/asm/entry/32: Replace RESTORE_RSI_RDI[_RDX] with
 open-coded 32-bit reads

On Tue, Jun 9, 2015 at 11:54 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> This doesn't change much, but this uses shorter 32-bit insns:
>
>         -48 8b 74 24 68         mov    0x68(%rsp),%rsi
>         -48 8b 7c 24 70         mov    0x70(%rsp),%rdi
>         -48 8b 54 24 60         mov    0x60(%rsp),%rdx
>         +8b 74 24 68            mov    0x68(%rsp),%esi
>         +8b 7c 24 70            mov    0x70(%rsp),%edi
>         +8b 54 24 60            mov    0x60(%rsp),%edx
>
> Since these are the only uses of RESTORE_RSI_RDI[_RDX], drop these macros.
>

It probably doesn't matter for these fast paths, but, for the full
slow path return, we really do need to restore the full pt_regs.
After all, the syscall we're returning from might be sigreturn.

--Andy
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