[Xen-devel] Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Zachary Amsden wrote:
> What about removing the identity functions entirely.  They are useless,
> really.  All that is needed is a patch site filled with nops for Xen to
> overwrite, just stuffing the value into the proper registers.  For
> 64-bit, it can be a simple mov to satisfy the constraints.
>   
I think it comes to the same thing really.  Both end up generating a 
series of nops with values entering and leaving in well-defined 
registers.  The x86-64 calling convention is a bit awkward because the 
first arg is in rdi and the ret is rax, so it can't quite be pure nops, 
or we use a non-standard calling-convention with appropriate thunks to 
call into C code.  I think a mov is a better performance-complexity 
tradeoff.
> > Also, I just posted patches to get rid of all pvops calls when fetching 
> > or setting flags in a pte, which I hope will help.
> >     
> Sounds like it will help.
>   
...but apparently not.

   J

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