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

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Date:	Sun, 14 Jun 2015 17:21:18 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	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 4/5] x86/asm/entry/32: Replace RESTORE_RSI_RDI[_RDX] with
 open-coded 32-bit reads

On 06/14/2015 10:40 AM, Ingo Molnar wrote:
> 
> * Denys Vlasenko <dvlasenk@...hat.com> wrote:
> 
>> 	+8b 74 24 68            mov    0x68(%rsp),%esi
>> 	+8b 7c 24 70            mov    0x70(%rsp),%edi
>> 	+8b 54 24 60            mov    0x60(%rsp),%edx
> 
> Btw., could you (in another patch) order the restoration properly, by pt_regs 
> memory order, where possible?

Will do.

> So this:
> 
>> +	movl	RSI(%rsp), %esi
>> +	movl	RDI(%rsp), %edi
>> +	movl	RDX(%rsp), %edx
>>  	movl	RIP(%rsp), %ecx
>>  	movl	EFLAGS(%rsp), %r11d
> 
> would become:
> 
> 	movl	RDX(%rsp), %edx
> 	movl	RSI(%rsp), %esi
> 	movl	RDI(%rsp), %edi
> 	movl	RIP(%rsp), %ecx
>  	movl	EFLAGS(%rsp), %r11d
> 
> ... or so.

Actually, ecx and r11 need to be loaded first. They are not so much "restored"
as "prepared for SYSRET insn". Every cycle lost in loading these
delays SYSRET. Other loads can be reordered by CPU past SYSRET.

> In fact I'd suggest we structure movl based restoration precisely after the field 
> order in the structure, and comment on cases where we depart from what's in 
> pt_regs - to make it all easier to verify.

Makes sense.

I'm sending a two-patch series which (1) open-codes RESTORE_RSI_RDI
and (2) sprinkles comments in this area.

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