linux-kernel - Re: [PATCH] x86-64: Fix CFI data for common

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Date:	Wed, 22 Feb 2012 08:05:35 +0000
From:	"Jan Beulich" <jbeulich@...e.com>
To:	<mjw@...hat.com>
Cc:	<fweisbec@...il.com>, <x86@...nel.org>, <tglx@...utronix.de>,
	<ak@...ux.intel.com>, <mingo@...hat.com>,
	<linux-kernel@...r.kernel.org>, <hpa@...or.com>
Subject: Re: [PATCH] x86-64: Fix CFI data for common_interrupt

>>> Mark Wielaard <mjw@...hat.com> 02/21/12 11:08 PM >>>
>On Tue, Feb 21, 2012 at 03:26:30PM +0000, Jan Beulich wrote:
>> >>> On 21.02.12 at 15:43, Mark Wielaard <mjw@...hat.com> wrote:
>> > For DW_CFA_def_register DWARF4 explicitly says so: "This operation is
>> > valid only if the current CFA rule is defined to use a register and
>> > offset." So one needs to use CFI_DEF_CFA with both a register and an
>> > offset here after the def_cfa_expression.
>> 
>> Hmm, that's in contrast to the gas implementation (but I'd certainly
>> give the specification preference if it explicitly states so, so gas
>> should at least emit a warning here rather than considering this
>> valid).
>
>I am afraid gas cannot help us here. Since like you pointed out in your
>patch:
>
>This requires the use of .cfi_escape (allowing arbitrary byte
>streams to be emitted into .eh_frame), as there is no
>.cfi_def_cfa_expression (which also cannot reasonably be
>expected, as it would require a full expression parser).
>
>So we are on our own here.

Hmm, yes, probably it wouldn't be nice if gas reset all its state when
.cfi_escape is used.

>> But provided the specification mandates this, I'm okay with the change
>> in principle. Just that specifying an offset of 0 doesn't look right then.
>
>Yeah, I dunno what I was thinking. The offset should be set to the offset
>that was there before when rsi was pushed. The attached patch does that
>by using the same value as was used at the start of common_interrupt.
>Does that look OK?

I would have thought that it should be SS+8-RBP (as %rbp is at the top
of the stack at that point). I can't verify this immediately, though, as I'm
not in the office today.

Jan

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