Re: [PATCH] x86/amd: Convert wrmsr_amd_safe() to use asm goto()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 04/04/2025 8:48 am, Jan Beulich wrote:
> On 03.04.2025 19:57, Andrew Cooper wrote:
>> Bloat-o-meter reports:
>>
>>   add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-29 (-29)
>>   Function                                     old     new   delta
>>   _probe_mask_msr                               99      94      -5
>>   init_amd                                    2418    2394     -24
>>
>> but that's because .fixup doesn't contain sized/typed symbols.  This also
>> drops two "mov -EFAULT, %reg; jmp ...;" sequences too.  The net saving is 
>> -50.
>>
>> wrmsr_amd_safe()'s return value is only checked against 0 (if at all), and
>> because of this, the compiler can now avoid manifesting 0/-EFAULT entirely,
>> and the %[fault] label simply lands on the right basic block.
>>
>> Convert to Xen style while rewriting.
>>
>> No functional change.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> Slightly RFC.  We use -EIO elsewhere for this purpose, and nothing in this
>> logic cares.
> {rd,wr}msr_safe() both use -EFAULT. What's "elsewhere" here?

Oh, that would be Linux.  Sorry, too much time spent staring at the same
logic in different codebases.

>
>> I was pleasently surprised by the manifestation of -EFAULT going away
>> entirely.
> I fear I don't understand this, given the -EFAULT is still there in the
> new code.

I meant about what the optimiser can do.  It hadn't occurred to me that
that was a valid transformation.

>
> Irrespective of these remarks:
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

~Andrew