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

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <keir@xxxxxxxxxxxxx> 20.06.07 17:16 >>>
>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.

Neither did I, except (obviously) on machines having SSE, but not SSE2 (which
was what I considered applying this for, with the additional need to tweak
idle vcpu handling so that in that context [where FP context altering doesn't
matter, as long as the rest of the system's aware of it] it could then be used).

>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.

Fine by me - I tried to say this in the description.

Jan


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