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Re: [Xen-devel] mmx sse emulation

To: Ashish Bijlani <ashish.bijlani@xxxxxxxxx>
Subject: Re: [Xen-devel] mmx sse emulation
From: Andre Przywara <andre.przywara@xxxxxxx>
Date: Wed, 5 Nov 2008 10:59:09 +0100
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Ashish Bijlani wrote:
Hi,

I want to emulate mmx/sse for hvm guests when applications inside hvm
guests are compiled for mmx/sse but the underlying hardware doesn't
support mmx/sse.
First: HVM guests require a virtualization capable processor. AFAIK all these processors support at least SSE2 (if not SSE3). So why do you want to emulate these instructions? Second: Applications should check the CPUID bit before using instruction set extension. So, if the host processor does not support MMX/SSE, the guest shouldn't see this bit, too. And I doubt that you are faster with emulating SSE compared to legacy x87-FPU executed natively.

So, what is the use-case of your proposal? Or am I missing something here?

Regards,
Andre.

> What is the best place to do this? i'm looking at
x86_emulate but i dunno if that is the best place to put the emulation
layer. any suggestions?? also, currently movq emulation is present in
x86_emulate for handling mmio. however, i realized that get_fpu fails
if the hardware doesn't have mmx capability. is it true or am i
missing something here?

Thanks,
Ashish
--
Andre Przywara
AMD-OSRC (Dresden)
Tel: x84917


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