Re: [Xen-devel] [PATCH] x86: introduce specialized clear_page()

[Keir Fraser <keir@xxxxxxxxxxxxx>]

On 19/6/07 11:16, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> While the patch also adds an SSE version, this is is currently orphaned
> as I am not certain about the benefit of special casing idle VCPUs in
> a few places (during context switching), so that at least in that
> context using %xmmN registers would be possible without crashing and/or
> corrupting guest state. The benefit of adding such support could be to
> reduce scheduling latency when a VCPU is to transition out of idle, but
> is busy doing page cleaning.

I measure no benefit from the XMM version (nor from upgrading the SSE2
version on x86/64 to clear from %rax instead of %eax). I guess this is
because the move is non-temporal and it's actually quite easy to max the
memory bandwidth.

So I stripped out the xmm version (it can't work anyway without some
kernel_fpu_begin/end kind of mechanism) and made the sse2 version common
across i386 and x86_64 since that was easily done and increases code
sharing.

 -- Keir


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