RE: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:mst@redhat.com]
> Sent: Thursday, April 18, 2013 3:48 AM
> To: KY Srinivasan
> Cc: Jan Beulich; olaf@aepfle.de; bp@alien8.de; apw@canonical.com;
> x86@kernel.org; tglx@linutronix.de; devel@linuxdriverproject.org;
> gregkh@linuxfoundation.org; jasowang@redhat.com; linux-
> kernel@vger.kernel.org; hpa@zytor.com
> Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> 
> On Wed, Apr 17, 2013 at 04:28:36PM +0000, KY Srinivasan wrote:
> >
> >
> > > -----Original Message-----
> > > From: Jan Beulich [mailto:JBeulich@suse.com]
> > > Sent: Wednesday, April 17, 2013 11:51 AM
> > > To: KY Srinivasan; Michael S. Tsirkin
> > > Cc: olaf@aepfle.de; bp@alien8.de; apw@canonical.com; x86@kernel.org;
> > > tglx@linutronix.de; devel@linuxdriverproject.org;
> gregkh@linuxfoundation.org;
> > > jasowang@redhat.com; linux-kernel@vger.kernel.org; hpa@zytor.com
> > > Subject: RE: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> > >
> > > >>> On 17.04.13 at 17:31, KY Srinivasan <kys@microsoft.com> wrote:
> > > > If Xen were to change where it would not unconditionally emulate Hyper-V,
> I
> > > > would not be opposed to taking
> > > > this check out.
> > >
> > > But it doesn't do this unconditionally, only upon admin request.
> >
> > >From the discussion we had a couple of months ago, the default setting was to
> enable
> > Hyper-V emulation for all guests. If this is not the case, we ought to be able to
> drop this.
> > However, I think it is not reasonable to add additional checks (in addition to
> hypervisor check)
> > to customize the run-time in the guest for the specific Hypervisor.
> >
> > Regards,
> >
> > K. Y
> 
> Parse error. What are you trying to say? I'm just saying it's best to do
> things in the way that make it possible for Xen to implement hyperv in a
> more complete way in the future and have things just work
> and in a way that does not change guest/hypervisor interface.

There is no error to parse here. I need to setup the vmbus interrupt before I
can contact the host. As I said I am fine with dropping this check if Xen behavior is
changed appropriately. We have a mechanism for hypervisor detection and a 
mechanism for discovering features within a hypervisor. Once we confirm the
existence of a specific hypervisor, we ought to be able to do what is needed for
that hypervisor (unless the hypervisor feature flag precludes it) without having to
go through hoops to discover if we are in a hypervisor emulation mode where the emulation
may not be complete. In this particular case, the need for VMBUS related infrastructure cannot be
currently determined via the feature detection mechanism. 

For features that cannot be detected via CPUID feature bits, we can use some other CPUID based
mechanism to discriminate. For instance in the case under discussion, I could  install
hyperv_callback_vector only if we are on an environment that supports the vmbus infrastructure.
Unfortunately there is no positive test for this. The best we can do is a negative test. I could move the 
Xen check and conditionally install the hyperv_callback vector. 

Regards,

K. Y
 
 
> 
> > >
> > > Jan
> > >
> > >
> >
>