Re: [Xen-devel] mmx sse emulation

["Ashish Bijlani" <ashish.bijlani@xxxxxxxxx>]

Hi Andre,

You are absolutely right. All hardware virtualization capable machines
have recent simd technology built-in. However, I'm just trying to
evaluate a case when HVM guests rely on the virtual hardware platform
and not on the actual hardware platform. Precisely, what would be the
performance gain/loss if hypervisor has to emulate the functionality.

-Ashish

On Wed, Nov 5, 2008 at 4:59 AM, Andre Przywara <andre.przywara@xxxxxxx> wrote:
> 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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