Re: [Xen-devel] [PATCH 0/5] x86: improve PDX <-> PFN and alike translations

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.02.18 at 17:47, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 28/02/18 13:51, Jan Beulich wrote:
>> 1: remove page.h and processor.h inclusion from asm_defns.h
>> 2: use PDEP for PTE flags insertion when available
>> 3: use PDEP/PEXT for maddr/direct-map-offset conversion when available
>> 4: use PDEP/PEXT for PFN/PDX conversion when available
>> 5: use MOV for PFN/PDX conversion when possible
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>
> 
> Ah - so this was the series you were on about which would have an
> interesting time in combination with my nop autosizing.
> 
> Do you have performance numbers for these changes?  I can certainly see
> the attraction of using BMI2 when available, but do the associated costs
> on incompatible hardware worth it?  I'm thinking specifically of turning
> all this inline bit manipulation into function calls?  (I genuinely
> don't know the answer, and it might be entirely fine, but I'm concerned
> about whether it may not be).

Btw, before you voice any performance concerns for older
hardware, please take into consideration the last patch of the
series, which converts the CALL to MOV on virtually all
hardware (as mentioned on some older thread I'm not sure the
hardware/firmware that this PDX/PFN conversion was written
for has ever made it to any customers).

Jan


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