Re: [PATCH][GIT PULL][v2.6.32] tracing/x86: Add check to detect GCC messing with mcount prologue

[H. Peter Anvin]

On 11/24/2009 09:12 AM, Andrew Haley wrote:
>>
>> If we're changing gcc anyway, then let's add the option of intercepting
>> the function at the point where the machine state is well-defined by
>> ABI, which is before the function stack frame is set up.
> 
> Hmm.  On the x86 I suppose we could just inject a naked call instruction,
> but not all aeches allow us to call anything before we've saved the return
> address.  Or are you talking x86 only?
> 

For x86, we should use a naked call.

For architectures where that is not possible, we should use a minimal
sequence such that the ABI state at the invocation point is 100% derivable.

On MIPS, for example, we could use a sequence such as:

	mov at, ra
	jal __fentry__

It would be up to __fentry__ to save the value in at and to restore it
back into ra before resuming, meaning that __fentry__ has a nonstandard
calling convention.

	-hpa
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