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 <[email protected]>
> Reviewed-by: Jan Beulich <[email protected]>
>
>> 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).
Are you happy for your R-by to stand, given this adjustment?
~Andrew
