Re: [Xen-devel] x86info: dump kvm cpuid's

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Tue, May 01, 2012 at 11:29:04AM +0100, Ian Campbell wrote:
> On Mon, 2012-04-30 at 10:38 +0100, Michael S. Tsirkin wrote:
> > On Mon, Apr 30, 2012 at 11:43:19AM +0300, Gleb Natapov wrote:
> > > On Sun, Apr 29, 2012 at 01:10:21PM +0300, Michael S. Tsirkin wrote:
> > > > The following makes 'x86info -r' dump kvm cpu ids
> > > > (signature+features) when running in a vm.
> > > > 
> > > > On the guest we see the signature and the features:
> > > > eax in: 0x40000000, eax = 00000000 ebx = 4b4d564b ecx = 564b4d56 edx = 
> > > > 0000004d
> > > > eax in: 0x40000001, eax = 0100007b ebx = 00000000 ecx = 00000000 edx = 
> > > > 00000000
> > > > 
> > > > On the host it just adds a couple of zero lines:
> > > > eax in: 0x40000000, eax = 00000000 ebx = 00000000 ecx = 00000000 edx = 
> > > > 00000000
> > > > eax in: 0x40000001, eax = 00000000 ebx = 00000000 ecx = 00000000 edx = 
> > > > 00000000
> > > > 
> > > This is too KVM specific.
> > 
> > That's what I have. I scratch my own itch.
> > 
> > > Other hypervisors may use more cpuid leafs.
> > 
> > But not less so no harm's done.
> > 
> > > As far as I see Hyper-V uses 5 and use cpuid.0x40000000.eax as max cpuid
> > > leaf available. Haven't checked Xen or VMWare.
> 
> Xen does the same, documentation in the Xen public interfaces header:
> http://xenbits.xen.org/docs/unstable/hypercall/include,public,arch-x86,cpuid.h.html.

So ack to my patch?

> If compat mode for another h/v is enabled then those leaves will appear
> at 0x40000000 and Xen's will be bumped up, so a fully Xen aware set of
> drivers (or detection routine, etc) should check at 0x100 intervals
> until 0x40010000 for the appropriate signatures (I realise that the docs
> are somewhat lacking in this regard, I should cook up a patch).
> 
> Ian.

How does guest know that the data at 0x40000100 makes sense?

-- 
MST

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