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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