mmx sse emulation

mmx sse emulation

[Ashish Bijlani]

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. 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

Re: mmx sse emulation

[Keir Fraser]

On 5/11/08 00:34, "Ashish Bijlani" <ashish.bijlani@gmail.com> wrote:

> 
> 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. 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?

x86_emulate() does its work by using the actual underlying processor
instructions (really it is a system emulator rather than an instruction
emulator). This wouldn't necessarily stop you adding full emulation for
certain instructions like SSE/MMX though, and it's probably the right place
to put that sort of thing.

 -- Keir

Re: mmx sse emulation

[Ashish Bijlani]

Hi Keir,

Thanks for the help. I've one more doubt though.

Calling x86_emulate from do_device_not_available after validating the
EM flag would work? Of course provided MMX/SS emulation code reside in
x86_emulate. Please lemme know.

Thanks,
Ashish

On Wed, Nov 5, 2008 at 2:34 AM, Keir Fraser <keir.fraser@eu.citrix.com> wrote:
> On 5/11/08 00:34, "Ashish Bijlani" <ashish.bijlani@gmail.com> wrote:
>
>>
>> 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. 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?
>
> x86_emulate() does its work by using the actual underlying processor
> instructions (really it is a system emulator rather than an instruction
> emulator). This wouldn't necessarily stop you adding full emulation for
> certain instructions like SSE/MMX though, and it's probably the right place
> to put that sort of thing.
>
>  -- Keir
>
>
>

Re: mmx sse emulation

[Keir Fraser]

On 5/11/08 07:52, "Ashish Bijlani" <ashish.bijlani@gmail.com> wrote:

> Hi Keir,
> 
> Thanks for the help. I've one more doubt though.
> 
> Calling x86_emulate from do_device_not_available after validating the
> EM flag would work? Of course provided MMX/SS emulation code reside in
> x86_emulate. Please lemme know.

Sounds about right to me.

 -- Keir

Re: mmx sse emulation

[Andre Przywara]

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

Re: mmx sse emulation

[Ashish Bijlani]

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@amd.com> 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
>
>

Re: mmx sse emulation

[Ashish Bijlani]

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@gmail.com> 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.
>
> -Ashish
>
> On Wed, Nov 5, 2008 at 4:59 AM, Andre Przywara <andre.przywara@amd.com> 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
>>
>>
>

Re: mmx sse emulation

[Ashish Bijlani]

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@planetcatfish.com> wrote:
> 2008/11/5 Ashish Bijlani <ashish.bijlani@gmail.com>
>>
>> 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@gmail.com>
>> 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@amd.com>
>> 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
>>>
>>>
>>
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
>
>