Re: [Xen-devel] [PATCHv2] x86info: dump kvm cpuid's

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

On Tue, May 01, 2012 at 01:27:05PM +0100, Ian Campbell wrote:
> On Mon, 2012-04-30 at 17:38 +0300, Michael S. Tsirkin wrote:
> > The following makes 'x86info -r' dump hypervisor leaf cpu ids
> > (for kvm this is 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
> > 
> > Hypervisor flag is checked to avoid output changes when not
> > running on a VM.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > 
> > Changes from v1:
> >     Make work on non KVM hypervisors (only KVM was tested).
> >     Avi Kivity said kvm will in the future report
> >     max HV leaf in eax. For now it reports eax = 0
> >         so add a work around for that.
> > 
> > ---
> > 
> > diff --git a/identify.c b/identify.c
> > index 33f35de..a4a3763 100644
> > --- a/identify.c
> > +++ b/identify.c
> > @@ -9,8 +9,8 @@
> >  
> >  void get_cpu_info_basics(struct cpudata *cpu)
> >  {
> > -   unsigned int maxi, maxei, vendor, address_bits;
> > -   unsigned int eax;
> > +   unsigned int maxi, maxei, maxhv, vendor, address_bits;
> > +   unsigned int eax, ebx, ecx;
> >  
> >     cpuid(cpu->number, 0, &maxi, &vendor, NULL, NULL);
> >     maxi &= 0xffff;         /* The high-order word is non-zero on some 
> > Cyrix CPUs */
> > @@ -19,7 +19,7 @@ void get_cpu_info_basics(struct cpudata *cpu)
> >             return;
> >  
> >     /* Everything that supports cpuid supports these. */
> > -   cpuid(cpu->number, 1, &eax, NULL, NULL, NULL);
> > +   cpuid(cpu->number, 1, &eax, &ebx, &ecx, NULL);
> 
> You probably want to check ebx, ecx, edx for the signatures of the
> hypervisor's you are willing to support and which you know do something
> sane with eax?

Everyone puts the max leaf in eax - this is what hardware
CPUs do. So I think we can just interpret eax as such.
If someone wants to put broken info in cpuid,
send a patch to blacklist it.

> Also it would be something worth reporting in its own
> right?
> 
> BTW, according to arch/x86/include/asm/kvm_para.h unsurprisingly KVM has
> a signature too 'KVMKVMKVM'.
> 
> >     cpu->stepping = eax & 0xf;
> >     cpu->model = (eax >> 4) & 0xf;
> >     cpu->family = (eax >> 8) & 0xf;
> > @@ -29,6 +29,19 @@ void get_cpu_info_basics(struct cpudata *cpu)
> >  
> >     cpuid(cpu->number, 0xC0000000, &maxei, NULL, NULL, NULL);
> >     cpu->maxei2 = maxei;
> > +   if (ecx & 0x80000000) {
> > +           cpuid(cpu->number, 0x40000000, &maxhv, NULL, NULL, NULL);
> > +           /*
> > +            * KVM up to linux 3.4 reports 0 as the max hypervisor leaf,
> > +            * where it really means 0x40000001.
> 
> This is something where I definitely think you want to check the
> signature first.
> 
> Ian.

kvm lets users change the signature.
But worst case we print one useless line.
Better than not printing something potentially useful.

-- 
MST

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