Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

[Zachary Amsden]

On Thu, 2009-01-22 at 14:44 -0800, Jeremy Fitzhardinge wrote:

> I did have some patches to do that at one point.  If you set pgd_val = 
> paravirt_nop, then the patching machinery will completely nop out the 
> call site.  The problem is that it depends on the calling convention 
> using the same regs for the first arg and return - true for 32-bit, but 
> not 64.  We could fix that with identity functions which the patcher 
> recognizes and can replace with either pure nops or inline appropriate 
> register moves.

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.

> 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.

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