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

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Message-Id: <4F43C5B60200007800074392@nat28.tlf.novell.com>
Date:	Tue, 21 Feb 2012 15:26:30 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Mark Wielaard" <mjw@...hat.com>
Cc:	"Frederic Weisbecker (commit_signer:4/25=16%)" <fweisbec@...il.com>,
	"maintainer:X86 ARCHITECTURE..." <x86@...nel.org>,
	"Thomas Gleixner(maintainer:X86 ARCHITECTURE...)" 
	<tglx@...utronix.de>,
	"Andi Kleen(commit_signer:5/25=20%)" <ak@...ux.intel.com>,
	"commit_signer:11/25=44%)Ingo Molnar (maintainer:X86 ARCHITECTURE..." 
	<mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	"commit_signer:4/25=16%) H. Peter Anvin(maintainer:X86 ARCHITECTURE..." 
	<hpa@...or.com>
Subject: Re: [PATCH] x86-64: Fix CFI data for common_interrupt

>>> On 21.02.12 at 15:43, Mark Wielaard <mjw@...hat.com> wrote:
> On Tue, 2012-02-21 at 14:26 +0000, Jan Beulich wrote:
>> >>> On 21.02.12 at 15:06, Mark Wielaard <mjw@...hat.com> wrote:
>> > Commit eab9e6 "x86-64: Fix CFI data for interrupt frames" introduced
>> > a DW_CFA_def_cfa_expression in the SAVE_ARGS_IRQ macro. To later define
>> > the CFA using a simple register+offset rule both register and offset
>> > need to be supplied. Just using CFI_DEF_CFA_REGISTER leaves the offset
>> > undefined. So use CFI_DEF_CFA with reg+off explicitly at the end of
>> > common_interrupt.
>> 
>> NAK, unless you can prove a path via which the offset will remain
>> unset until hitting a CFI_DEF_CFA_REGISTER. And if you indeed
>> found such a path, the entry point of the path is where the problem
>> ought to be fixed.
>> 
>> Are you perhaps thinking that .cfi_def_cfa_register invalidates
>> the offset in any way? That, to my knowledge, isn't the case, it
>> just replaces the CFA register with the one specified, leaving the
>> offset unchanged.
> 
> DW_CFA_def_cfa_expression invalidates the offset (and register). Used
> through the interrupt macro for do_IRQ which uses the SAVE_ARGS_IRQ to
> define common_interrupt. So after using DW_CFA_def_cfa_expression we get
> a CFI_DEF_REGISTER and the CFI for common_interrupt looks like:
> 
>  [  6e30] FDE length=148 cie=[  6e18]
>    CIE_pointer:              28184
>    initial_location:         0xffffffff815e8d00 <common_interrupt>
>    address_range:            0x1ba
> 
>    Program:
> [...]
>      advance_loc 1 to 0x69
>      def_cfa_expression 6
>           [   0] breg7 0
>           [   2] deref
>           [   3] const1u 136
>           [   5] plus
>      advance_loc 22 to 0x7f
>      def_cfa_register r4 (rsi)
> [...]
> 
> 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).

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.

Jan

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