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

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Message-ID: <CALCETrU5aA6yLjF-PUcJUkqcGftL=Vp_WiKUUrYTmm8+3VQdVQ@mail.gmail.com>
Date:	Tue, 24 Feb 2015 14:55:31 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.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 4/4] x86: save user %rsp in pt_regs->sp

On Tue, Feb 24, 2015 at 10:51 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
> PER_CPU(old_rsp) usage is simplified - now it is used only
> as temp storage, and userspace stack pointer is immediately stored
> in pt_regs->sp on syscall entry, instead of being used later,
> on syscall exit. This allows to get rid of thread_struct::usersp.

I'm not seeing any performance loss here, which is good.  This is a
nice cleanup.

> @@ -253,11 +247,13 @@ GLOBAL(system_call_after_swapgs)
>          */
>         ENABLE_INTERRUPTS(CLBR_NONE)
>         ALLOC_PT_GPREGS_ON_STACK 6*8 /* 6*8: space for orig_ax and iret frame */
> +       movq    %rcx,RIP(%rsp)
> +       movq    %r11,EFLAGS(%rsp)

> +       movq    PER_CPU_VAR(old_rsp),%rcx
> +       movq    %rcx,RSP(%rsp)

It's a bit unfortunate that this adds two instructions instead of just
one.  I guess this gets fix when we start using push instead of mov
here.  (And, if we use push, then we can fix up the RCX slot for free,
too.)

Acked-by me, but I won't apply it until v2 to avoid messy rebases.

--Andy
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