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).
> 
> What generation of binutils do you expect this all to work with?

So one question here is whether to make this independent of
binutils version, and instead make use of gcc's new V operand
modifier (which didn't exist yet back when I wrote this). Since
those indirect thunk patches are likely to be backported by
distros, I'd expect us to be able to use the more flexible
variant of the alternatives here in a wider set of cases if we
went that route.

Jan


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