Re: [Xen-devel] mmx sse emulation

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

in an attempt to emulate mmx/sse instructions from within xen, i tried
setting EM and NE bit in CR0 to have mmx/sse instructions fault into
xen. however, the hvm guest doesn't boot after that. also, i don't get
any "device_not_available_fault" any ideas what could be wrong? am i
missing something?

this is what i've done :-

int hvm_set_cr0(unsigned long value) {
...
value &= ~HVM_CR0_GUEST_RESERVED_BITS;

value |= (X86_CR0_EM | X86_CR0_NE); // EMULATE FPU

/* ET is reserved and should be always be 1. */
    value |= X86_CR0_ET;
...
}

I run system with dom0 4 vcpus on 0-2 cpu cores and hvm 1 vcpu on the
3rd cpu core

-Ashish
On Wed, Nov 5, 2008 at 10:10 AM, mats petersson <mats@xxxxxxxxxxxxxxxxx> wrote:
> 2008/11/5 Ashish Bijlani <ashish.bijlani@xxxxxxxxx>
>>
>> If I'm not wrong, Bochs emulates SSE/MMX instructions and qemu uses
>> dynamic translation. Does this mean that I use SSE/MMX emulation code
>> from Bochs to put under x86_emulate? or am I missing something?
>> Thanks.
>>
>> On Wed, Nov 5, 2008 at 5:34 AM, Ashish Bijlani <ashish.bijlani@xxxxxxxxx>
>> wrote:
>> > 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.
>
> The performance loss would be HUGE. First of all, you'd fall into the
> hypervisor and get back out again, which takes a fair few cycles (like more
> than 100x or more that of a single instruction). Then you have to emulate
> the actual instruction itself, which will be the small part of the ovehead.
>
> Many SSE/MMX/3DNow! instructions execute in 1-2 clockcycles. My guess would
> be that it would reduce any SSE optimized code to a crawl (like in the order
> of 100-1000x slower). Such an application would be much better off running
> in x87 mode.
>
> I was looking at doing SSE/MMX emulation in x86_emulate a long time ago, but
> the purpose of that wasn't to emulate SSE/MMX as such, but rather allow
> SSE/MMX to access emulated hardware (such as video memory).
>
> --
> Mats
>>
>> -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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