Re: [PATCH v2] x86/hvm: Use for_each_set_bit() in hvm_emulate_writeback()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.03.2025 14:34, Andrew Cooper wrote:
> On 17/03/2025 9:09 am, Jan Beulich wrote:
>> On 14.03.2025 21:49, Andrew Cooper wrote:
>>> ... which is more consise than the opencoded form, and more efficient when
>>> compiled.
>>>
>>> For production VMs, ~100% of emulations are simple MOVs, so it is likely 
>>> that
>>> there are no segments to write back.
>>>
>>> Furthermore, now that find_{first,next}_bit() are no longer in use, the
>>> seg_reg_{accessed,dirty} fields aren't forced to be unsigned long, although
>>> they do need to remain unsigned int because of __set_bit() elsewhere.
>>>
>>> No functional change.
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
>>> I still can't persuade GCC to do the early exit prior to establishing the
>>> stack frame, and unlike do_livepatch_work(), it's not critical enough to
>>> require noinline games.
>> Then is ...
>>
>>> --- a/xen/arch/x86/hvm/emulate.c
>>> +++ b/xen/arch/x86/hvm/emulate.c
>>> @@ -3022,18 +3022,16 @@ void hvm_emulate_init_per_insn(
>>>  void hvm_emulate_writeback(
>>>      struct hvm_emulate_ctxt *hvmemul_ctxt)
>>>  {
>>> -    enum x86_segment seg;
>>> +    struct vcpu *curr;
>>> +    unsigned int dirty = hvmemul_ctxt->seg_reg_dirty;
>>>  
>>> -    seg = find_first_bit(&hvmemul_ctxt->seg_reg_dirty,
>>> -                         ARRAY_SIZE(hvmemul_ctxt->seg_reg));
>>> +    if ( likely(!dirty) )
>>> +        return;
>> ... this worthwhile at all? I'm surprised anyway that I see you use likely()
>> here, when generally you argue against its use.
> 
> No, it's not worth it.  In fact, simplifying makes the function smaller.
> 
> void hvm_emulate_writeback(
>     struct hvm_emulate_ctxt *hvmemul_ctxt)
> {
>     struct vcpu *curr = current;
>     unsigned int dirty = hvmemul_ctxt->seg_reg_dirty;
> 
>     for_each_set_bit ( seg, dirty )
>         hvm_set_segment_register(curr, seg, &hvmemul_ctxt->seg_reg[seg]);
> }
> 
> gets a bloat-o-meter score of 131 down to 72 (-59).

That's surprisingly much.

> Are you happy for your R-by to stand, given this adjustment?

Certainly.

Jan