Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

From: Jeremy Fitzhardinge
Date: Thu Jan 22 2009 - 19:14:22 EST

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.

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