Xen HVM cpuid problem

Xen HVM cpuid problem

[Christoph Egger]

Hi Keir,

For HVM guests, all cpuid Fn's going through domain_cpuid()
iterate over the loop and then return 0 for all four registers.

Guests OS's and cpuid tools in HVM which query for
cpuid Fn 0000.0000 %eax and 8000.0000 %eax, see the value 0 and think,
Xen emulates oldish 386/486 CPUs.

This leads to strange boot failures, "your CPU does not support long mode"
or "minimum required CPU is a 486DX CPU" when launching 32bit or 64bit HVM 
guests.

To reproduce this, cpuid is NOT explicitely configured
in the guest config file.

BTW: Is there a reason for not handling cpuid in one central place?

Christoph


-- 
AMD Saxony, Dresden, Germany
Operating System Research Center

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   Dr. Hans-R. Deppe, Thomas McCoy

Re: Xen HVM cpuid problem

[Jean Guyader]

Christoph Egger wrote:
> Hi Keir,
> 
> For HVM guests, all cpuid Fn's going through domain_cpuid()
> iterate over the loop and then return 0 for all four registers.
> 

This loop return 0 to all four registers, if there is no cpuid 
configuration for this input.

> 
> To reproduce this, cpuid is NOT explicitely configured
> in the guest config file.

What do you would like to say by "NOT explicitly configured" ? Could you 
please give us an example ?

> 
> BTW: Is there a reason for not handling cpuid in one central place?
> 

Yes, there is, because cpuid havn't exactly the same behavior under 
intel and amd.

-- 
Jean Guyader

Re: Xen HVM cpuid problem

[Keir Fraser]

All HVM guests receive a default configuration from libxc's
xc_cpuid_apply_policy(). You must update your tools if you have updated your
hypervisor to recent xen-unstable. Otherwise you will indeed get bogus CPUID
behaviour.

 -- Keir

On 13/5/08 13:41, "Christoph Egger" <Christoph.Egger@amd.com> wrote:

> 
> Hi Keir,
> 
> For HVM guests, all cpuid Fn's going through domain_cpuid()
> iterate over the loop and then return 0 for all four registers.
> 
> Guests OS's and cpuid tools in HVM which query for
> cpuid Fn 0000.0000 %eax and 8000.0000 %eax, see the value 0 and think,
> Xen emulates oldish 386/486 CPUs.
> 
> This leads to strange boot failures, "your CPU does not support long mode"
> or "minimum required CPU is a 486DX CPU" when launching 32bit or 64bit HVM
> guests.
> 
> To reproduce this, cpuid is NOT explicitely configured
> in the guest config file.
> 
> BTW: Is there a reason for not handling cpuid in one central place?
> 
> Christoph
> 

Re: Xen HVM cpuid problem

[Christoph Egger]

On Tuesday 13 May 2008 14:56:00 Keir Fraser wrote:
> All HVM guests receive a default configuration from libxc's
> xc_cpuid_apply_policy(). You must update your tools if you have updated
> your hypervisor to recent xen-unstable. Otherwise you will indeed get bogus
> CPUID behaviour.

Tnx. Updating the tools fixed it. I should get used on updating the tools more 
regularly. Sorry for the noise.

Christoph

>
>  -- Keir
>
> On 13/5/08 13:41, "Christoph Egger" <Christoph.Egger@amd.com> wrote:
> > Hi Keir,
> >
> > For HVM guests, all cpuid Fn's going through domain_cpuid()
> > iterate over the loop and then return 0 for all four registers.
> >
> > Guests OS's and cpuid tools in HVM which query for
> > cpuid Fn 0000.0000 %eax and 8000.0000 %eax, see the value 0 and think,
> > Xen emulates oldish 386/486 CPUs.
> >
> > This leads to strange boot failures, "your CPU does not support long
> > mode" or "minimum required CPU is a 486DX CPU" when launching 32bit or
> > 64bit HVM guests.
> >
> > To reproduce this, cpuid is NOT explicitely configured
> > in the guest config file.
> >
> > BTW: Is there a reason for not handling cpuid in one central place?
> >
> > Christoph



-- 
AMD Saxony, Dresden, Germany
Operating System Research Center

Legal Information:
AMD Saxony Limited Liability Company & Co. KG
Sitz (Geschäftsanschrift):
   Wilschdorfer Landstr. 101, 01109 Dresden, Deutschland
Registergericht Dresden: HRA 4896
vertretungsberechtigter Komplementär:
   AMD Saxony LLC (Sitz Wilmington, Delaware, USA)
Geschäftsführer der AMD Saxony LLC:
   Dr. Hans-R. Deppe, Thomas McCoy