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Date:	Tue, 12 Aug 2014 11:31:07 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	"H. Peter Anvin" <hpa@...or.com>, Jan Beulich <JBeulich@...e.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	X86 ML <x86@...nel.org>, Alexei Starovoitov <ast@...mgrid.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct
 pt_regs"

On 08/11/2014 05:13 PM, H. Peter Anvin wrote:
> On 08/11/2014 08:08 AM, Jan Beulich wrote:
>>> No, in *human language*.  What does the DW_CFA_def_cfa_expression
>>> actually aim to accomplish?  If you don't know the innards of the DWARF
>>> spec, the whole thing might as well be Hungarian.
>>
>> Just like the other DW_CFA_def_cfa_* ones it sets the current
>> frame address (CFA), just not via one of the pre-canned shortcuts,
>> but via an expression (in the case here de-referencing the stack
>> pointer to read the top of stack, and then adding the necessary
>> offset). So it indeed is similar enough to other .cfi_* annotations we
>> use without further comments.
>>
> 
> Actually, what you had inside the parenteses there is actually a
> half-decent comment.  I'm going to pretend the rest of this wasn't posted.

Jan, Pater, does this look correct _and_ human-understandable?

--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -652,10 +652,14 @@ END(interrupt)
        cmovzq PER_CPU_VAR(irq_stack_ptr),%rsp
        CFI_DEF_CFA_REGISTER    rsi
        pushq %rsi
+       /*
+        * For debugger:
+        * "CFA (Current Frame Address) is the value on stack + offset"
+        */
        CFI_ESCAPE      0x0f /* DW_CFA_def_cfa_expression */, 6, \
-                       0x77 /* DW_OP_breg7 */, 0, \
+                       0x77 /* DW_OP_breg7 (rsp) */, 0, \
                        0x06 /* DW_OP_deref */, \
-                       0x08 /* DW_OP_const1u */, SS+8-RBP, \
+                       0x08 /* DW_OP_const1u */, SIZEOF_PTREGS-RBP, \
                        0x22 /* DW_OP_plus */
        /* We entered an interrupt context - irqs are off: */
        TRACE_IRQS_OFF

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