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SubjectRe: lmbench lat_mmap slowdown with CONFIG_PARAVIRT
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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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