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

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Date:	Mon, 11 Aug 2014 09:40:35 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Frederic Weisbecker" <fweisbec@...il.com>,
	"Denys Vlasenko" <vda.linux@...glemail.com>
Cc:	"Andy Lutomirski" <luto@...capital.net>,
	"Kees Cook" <keescook@...omium.org>,
	"Will Drewry" <wad@...omium.org>, "X86 ML" <x86@...nel.org>,
	"Alexei Starovoitov" <ast@...mgrid.com>,
	"Denys Vlasenko" <dvlasenk@...hat.com>,
	"Oleg Nesterov" <oleg@...hat.com>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 4/5] x86: entry_64.S: always allocate complete
 "struct pt_regs"

>>> On 11.08.14 at 02:46, <fweisbec@...il.com> wrote:
> 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 :/

Each of its lines has a comment; with other CFI annotations not
each having comments, I don't see what else is needed here.

> I don't know, only Jan Beulich understands those CFI black magic.

That would be very said if true.

In any case: This needs to be a CFI_ESCAPE because there's no
other way I know of to emit the DW_CFA_def_cfa_expression.
And the change to it looks correct to me.

Jan

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