>>> On 10.02.16 at 16:03, <andrew.coop...@citrix.com> wrote:
> On 10/02/16 12:57, Jan Beulich wrote:
>> Also drop an unnecessary va adjustment in the code being touched.
>>
>> Signed-off-by: Jan Beulich <jbeul...@suse.com>
>>
>> --- a/xen/arch/x86/flushtlb.c
>> +++ b/xen/arch/x86/flushtlb.c
>> @@ -139,10 +139,12 @@ unsigned int flush_area_local(const void
>> c->x86_clflush_size && c->x86_cache_size && sz &&
>> ((sz >> 10) < c->x86_cache_size) )
>> {
>> - va = (const void *)((unsigned long)va & ~(sz - 1));
>> + alternative(ASM_NOP3, "sfence", X86_FEATURE_CLFLUSHOPT);
>
> Why separate? This would be better in the lower alternative(), with one
> single nop making up the difference in length. That way, processors
> without CLFLUSHOPT don't suffer the 1 cycle instruction decode stall
> from the redundant rex prefix.
Why would we want the fence inside the loop - a single fence is
sufficient for the entire flush.
Also if we're worried about the REX decode, this could easily be a
NOP instead, just that I'm not certain which one in the end is less
decode overhead.
Jan
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