linux-kernel - Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct pt

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Message-ID: <20140811004559.GA2656@lerouge>
Date:	Mon, 11 Aug 2014 02:46:01 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>, X86 ML <x86@...nel.org>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct
 pt_regs"

On Mon, Aug 04, 2014 at 05:03:42AM +0200, Denys Vlasenko wrote:
> On Sat, Aug 2, 2014 at 1:19 AM, Frederic Weisbecker <fweisbec@...il.com> wrote:
> >>       CFI_ESCAPE      0x0f /* DW_CFA_def_cfa_expression */, 6, \
> >>                       0x77 /* DW_OP_breg7 */, 0, \
> >>                       0x06 /* DW_OP_deref */, \
> >> -                     0x08 /* DW_OP_const1u */, SS+8-RBP, \
> >> +                     0x08 /* DW_OP_const1u */, SS+8, \
> >>                       0x22 /* DW_OP_plus */
> >>       /* We entered an interrupt context - irqs are off: */
> >>       TRACE_IRQS_OFF
> >> -
> >>       call \func
> >>       .endm
> >>
> >> @@ -749,10 +719,9 @@ ret_from_intr:
> >>
> >>       /* Restore saved previous stack */
> >>       popq %rsi
> >
> > And then you pop to rsi. Ok that indeed works but perhaps we should keep it symetrical
> > just for clarity? Any reason why we can't reuse rdi here?
> 
> I changed this entire area in v2: basically, I will not change the logic,
> but will add comments explaining what are we doing here, and why.
> (Some minor code changes will be done, not affecting the logic).
> 
> While we are at it, what this  CFI_ESCAPE thing does here?
> As usual, it has no comment :/

I don't know, only Jan Beulich understands those CFI black magic.
BTW he doesn't appears to be Cc, we should add him. 

> 
> -- 
> vda
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