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.coop...@citrix.com>
>> Reviewed-by: Jan Beulich <jbeul...@suse.com>
>>
>>> 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
