RE: [Xen-devel] A simple question of CPU emulation in Xen

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

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@xxxxxxxxxx
> > To: james.harper@xxxxxxxxxxxxxxxx
> > CC: tinnycloud@xxxxxxxxxxx; xen-devel@xxxxxxxxxxxxxxxxxxx
> > 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.


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