A simple question of CPU emulation in Xen

A simple question of CPU emulation in Xen

[MaoXiaoyun]

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Hi :
 
     I have a simple question on CPU emulation in Xen.
 
    Currently, inside guest, we can see that the CPU information incluing types and frequency is exactly
same as physical CPU. Is it possible for Xen(or qemu) to emulate a different type of CPU. That is inside 
guest, we see a different CPU types?
 
    If it is possible, could someone elighten me where to start.
 
   Many thanks. 		 	   		  

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Re: A simple question of CPU emulation in Xen

[Alan Cox]

On Fri, 13 May 2011 22:41:44 +0800
MaoXiaoyun <tinnycloud@hotmail.com> wrote:

> 
> Hi :
>  
>      I have a simple question on CPU emulation in Xen.
>  
>     Currently, inside guest, we can see that the CPU information incluing types and frequency is exactly
> same as physical CPU. Is it possible for Xen(or qemu) to emulate a different type of CPU. That is inside 
> guest, we see a different CPU types?

Qemu supports a huge range of CPU and platform emulations, www.qemu.org.
Most of this falls outside of Xen as Qemu doesn't need or benefit from
Xen or KVM when doing cross cpu emulation.

RE: A simple question of CPU emulation in Xen

[Dan Magenheimer]

> From: Alan Cox [mailto:alan@lxorguk.ukuu.org.uk]
> Subject: Re: [Xen-devel] A simple question of CPU emulation in Xen
> 
> On Fri, 13 May 2011 22:41:44 +0800
> MaoXiaoyun <tinnycloud@hotmail.com> wrote:
> 
> >
> > Hi :
> >
> >      I have a simple question on CPU emulation in Xen.
> >
> >     Currently, inside guest, we can see that the CPU information
> incluing types and frequency is exactly
> > same as physical CPU. Is it possible for Xen(or qemu) to emulate a
> different type of CPU. That is inside
> > guest, we see a different CPU types?
> 
> Qemu supports a huge range of CPU and platform emulations,
> www.qemu.org.
> Most of this falls outside of Xen as Qemu doesn't need or benefit from
> Xen or KVM when doing cross cpu emulation.

Also see "MagiXen: Combining Binary Translation and Virtualization"
here: http://www.hpl.hp.com/techreports/2007/HPL-2007-77.html 

RE: A simple question of CPU emulation in Xen

[James Harper]

> Hi :
> 
>      I have a simple question on CPU emulation in Xen.
> 
>     Currently, inside guest, we can see that the CPU information
incluing
> types and frequency is exactly
> same as physical CPU. Is it possible for Xen(or qemu) to emulate a
different
> type of CPU. That is inside
> guest, we see a different CPU types?
> 
>     If it is possible, could someone elighten me where to start.
> 

Do you want to emulate a completely different platform (eg PPC / alpha /
Sparc) or just pretend the cpu is a slightly different flavours (eg
running a late model Xeon CPU but emulate a P2)?

In a lot of cases you can do the latter by changing some CPUID values in
the DomU config to mask the CPU features so the guest thinks it is
running on an older model.

James

Re: A simple question of CPU emulation in Xen

[hotmaim]

在 2011-5-14，7:41，"James Harper" <james.harper@bendigoit.com.au> 写道：

>> Hi :
>> 
>>     I have a simple question on CPU emulation in Xen.
>> 
>>    Currently, inside guest, we can see that the CPU information
> incluing
>> types and frequency is exactly
>> same as physical CPU. Is it possible for Xen(or qemu) to emulate a
> different
>> type of CPU. That is inside
>> guest, we see a different CPU types?
>> 
>>    If it is possible, could someone elighten me where to start.
>> 
> 
> Do you want to emulate a completely different platform (eg PPC / alpha /
> Sparc) or just pretend the cpu is a slightly different flavours (eg
> running a late model Xeon CPU but emulate a P2)?
> 
Thanks, exactly what I want.
since cpus are shared along All VMs, so In guest, youn will get lower score if we make tests 
with Xeon.  While higher score is it is pretend to be P2.
> In a lot of cases you can do the latter by changing some CPUID values in
> the DomU config to mask the CPU features so the guest thinks it is
> running on an older model.

Could i have the configuration u refer to? In hvm file?
Thanks

> 
> James
> 

RE: A simple question of CPU emulation in Xen

[James Harper]

> 
> 在 2011-5-14，7:41，"James Harper" <james.harper@bendigoit.com.au> 写道：
> 
> >> Hi :
> >>
> >>     I have a simple question on CPU emulation in Xen.
> >>
> >>    Currently, inside guest, we can see that the CPU information
> > incluing
> >> types and frequency is exactly
> >> same as physical CPU. Is it possible for Xen(or qemu) to emulate a
> > different
> >> type of CPU. That is inside
> >> guest, we see a different CPU types?
> >>
> >>    If it is possible, could someone elighten me where to start.
> >>
> >
> > Do you want to emulate a completely different platform (eg PPC / alpha /
> > Sparc) or just pretend the cpu is a slightly different flavours (eg
> > running a late model Xeon CPU but emulate a P2)?
> >
> Thanks, exactly what I want.
> since cpus are shared along All VMs, so In guest, youn will get lower score if
> we make tests
> with Xeon.  While higher score is it is pretend to be P2.
> > In a lot of cases you can do the latter by changing some CPUID values in
> > the DomU config to mask the CPU features so the guest thinks it is
> > running on an older model.
> 
> Could i have the configuration u refer to? In hvm file?
> Thanks
> 

I did something like this to give my DomU the lowest common feature set between my AMD and Intel based servers for live migration:

cpuid = [ '0:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
          '1:eax=0x06b1,ecx=xxxxxxxxxxx0000xx00xxx0000000xx0,edx=xxx00000xxxxxxx0xxxxxxxxx0xxxxxx',
          '4:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
          '0x80000000:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0']

But I haven't tried it in a while so I can't tell if it still works as it should. I do know that Silverlight won't install with the above because it requires some feature that I've masked out.

There are some other examples in the example configs I think.

James

RE: A simple question of CPU emulation in Xen

[MaoXiaoyun]

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> Subject: RE: [Xen-devel] A simple question of CPU emulation in Xen
> Date: Mon, 16 May 2011 11:46:26 +1000
> From: james.harper@bendigoit.com.au
> To: tinnycloud@hotmail.com
> CC: xen-devel@lists.xensource.com
> 
> > 
> > 在 2011-5-14，7:41，"James Harper" <james.harper@bendigoit.com.au> 写道：
> > 
> > >> Hi :
> > >>
> > >> I have a simple question on CPU emulation in Xen.
> > >>
> > >> Currently, inside guest, we can see that the CPU information
> > > incluing
> > >> types and frequency is exactly
> > >> same as physical CPU. Is it possible for Xen(or qemu) to emulate a
> > > different
> > >> type of CPU. That is inside
> > >> guest, we see a different CPU types?
> > >>
> > >> If it is possible, could someone elighten me where to start.
> > >>
> > >
> > > Do you want to emulate a completely different platform (eg PPC / alpha /
> > > Sparc) or just pretend the cpu is a slightly different flavours (eg
> > > running a late model Xeon CPU but emulate a P2)?
> > >
> > Thanks, exactly what I want.
> > since cpus are shared along All VMs, so In guest, youn will get lower score if
> > we make tests
> > with Xeon. While higher score is it is pretend to be P2.
> > > In a lot of cases you can do the latter by changing some CPUID values in
> > > the DomU config to mask the CPU features so the guest thinks it is
> > > running on an older model.
> > 
> > Could i have the configuration u refer to? In hvm file?
> > Thanks
> > 
> 
> I did something like this to give my DomU the lowest common feature set between my AMD and Intel based servers for live migration:
> 
> cpuid = [ '0:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
> '1:eax=0x06b1,ecx=xxxxxxxxxxx0000xx00xxx0000000xx0,edx=xxx00000xxxxxxx0xxxxxxxxx0xxxxxx',
> '4:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
> '0x80000000:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0']
> 
> But I haven't tried it in a while so I can't tell if it still works as it should. I do know that Silverlight won't install with the above because it requires some feature that I've masked out.
> 
> There are some other examples in the example configs I think.
 
I've tried a little bit, it still works.
I'll take a look CPUID configuration in detail myself.
Many thanks.
> 
> James
 		 	   		  

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RE: A simple question of CPU emulation in Xen

[Ian Campbell]

> > Could i have the configuration u refer to? In hvm file?

> I did something like this to give my DomU the lowest common feature
> set between my AMD and Intel based servers for live migration:
> 
> cpuid = [ '0:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
>           '1:eax=0x06b1,ecx=xxxxxxxxxxx0000xx00xxx0000000xx0,edx=xxx00000xxxxxxx0xxxxxxxxx0xxxxxx',
>           '4:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
>           '0x80000000:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0']
> 

In addition to this xend syntax xl in 4.1 and later supports a more
friendly syntax, described in the commit message for 22217:27c01a2e2a47.

Ian.

> But I haven't tried it in a while so I can't tell if it still works as
> it should. I do know that Silverlight won't install with the above
> because it requires some feature that I've masked out.
> 
> There are some other examples in the example configs I think.

RE: A simple question of CPU emulation in Xen

[MaoXiaoyun]

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> Subject: RE: [Xen-devel] A simple question of CPU emulation in Xen
> From: Ian.Campbell@citrix.com
> To: james.harper@bendigoit.com.au
> CC: tinnycloud@hotmail.com; xen-devel@lists.xensource.com
> Date: Mon, 16 May 2011 09:04:15 +0100
> 
> 
> > > Could i have the configuration u refer to? In hvm file?
> 
> > I did something like this to give my DomU the lowest common feature
> > set between my AMD and Intel based servers for live migration:
> > 
> > cpuid = [ '0:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
> > '1:eax=0x06b1,ecx=xxxxxxxxxxx0000xx00xxx0000000xx0,edx=xxx00000xxxxxxx0xxxxxxxxx0xxxxxx',
> > '4:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
> > '0x80000000:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0']
> > 
> 
> In addition to this xend syntax xl in 4.1 and later supports a more
> friendly syntax, described in the commit message for 22217:27c01a2e2a47.
> 
> Ian.
> 
 
Thanks Ian.
 
Will these impact domU performance behavior if I give a diffrernet CPU type and othere configurations(such as frequency, doubt this is supported)?
 

> > But I haven't tried it in a while so I can't tell if it still works as
> > it should. I do know that Silverlight won't install with the above
> > because it requires some feature that I've masked out.
> > 
> > There are some other examples in the example configs I think.
> 
> 
 		 	   		  

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RE: A simple question of CPU emulation in Xen

[Ian Campbell]

On Thu, 2011-05-19 at 03:15 +0100, MaoXiaoyun wrote:
> 
>  
> > Subject: RE: [Xen-devel] A simple question of CPU emulation in Xen
> > From: Ian.Campbell@citrix.com
> > To: james.harper@bendigoit.com.au
> > CC: tinnycloud@hotmail.com; xen-devel@lists.xensource.com
> > Date: Mon, 16 May 2011 09:04:15 +0100
> > 
> > 
> > > > Could i have the configuration u refer to? In hvm file?
> > 
> > > I did something like this to give my DomU the lowest common
> feature
> > > set between my AMD and Intel based servers for live migration:
> > > 
> > > cpuid = [ '0:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
> > >
> '1:eax=0x06b1,ecx=xxxxxxxxxxx0000xx00xxx0000000xx0,edx=xxx00000xxxxxxx0xxxxxxxxx0xxxxxx',
> > > '4:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0',
> > > '0x80000000:eax=0x3,ebx=0x0,ecx=0x0,edx=0x0']
> > > 
> > 
> > In addition to this xend syntax xl in 4.1 and later supports a more
> > friendly syntax, described in the commit message for
> 22217:27c01a2e2a47.
>  
> Will these impact domU performance behavior if I give a diffrernet CPU
> type and othere configurations(such as frequency, doubt this is
> supported)?

This setting purely controls what is reported to the guest, it has no
impact on the actual functioning of the hardware.

However the guest may make decisions based on this information which
could impact performance. One example would be the selection of which
memcpy routine to use based on the availability of various generations
of SSE or other factors (I think Linux does this).

Ian.