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

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: devel [mailto:devel-bounces@linuxdriverproject.org] On Behalf Of KY
> Srinivasan
> Sent: Friday, February 01, 2013 10:11 AM
> To: Stefano Stabellini; H. Peter Anvin
> Cc: olaf@aepfle.de; gregkh@linuxfoundation.org; jasowang@redhat.com;
> x86@kernel.org; linux-kernel@vger.kernel.org; bp@alien8.de; Jan Beulich;
> apw@canonical.com; devel@linuxdriverproject.org; tglx@linutronix.de
> Subject: RE: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> 
> 
> 
> > -----Original Message-----
> > From: Stefano Stabellini [mailto:stefano.stabellini@eu.citrix.com]
> > Sent: Friday, February 01, 2013 8:20 AM
> > To: H. Peter Anvin
> > Cc: Jan Beulich; KY Srinivasan; 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
> > Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> >
> > On Thu, 31 Jan 2013, H. Peter Anvin wrote:
> > > On 01/30/2013 12:53 AM, Jan Beulich wrote:
> > > >
> > > > I'm not convinced that's the right approach - any hypervisor
> > > > could do similar emulation, and hence you either want to make
> > > > sure you run on Hyper-V (by excluding all others), or you
> > > > tolerate using the emulation (which may require syncing up with
> > > > the other guest implementations so that shared resources don't
> > > > get used by two parties).
> > > >
> > > > I also wonder whether using the Hyper-V emulation (where
> > > > useful, there might not be anything right now, but this may
> > > > change going forward) when no Xen support is configured
> > > > wouldn't be better than not using anything...
> > > >
> > >
> > > I'm confused about "the right approach" here is.  As far as I
> > > understand, this only can affect a Xen guest where HyperV guest support
> > > is enabled but not Xen support, and only because Xen emulates HyperV but
> > > does so incorrectly.
> > >
> > > This is a Xen bug, and as such it makes sense to reject Xen
> > > specifically.  If another hypervisor emulates HyperV and does so
> > > correctly there seems to be no reason to reject it.
> >
> > I don't think so.
> >
> > AFAIK originally there were features exported as flags and Xen doesn't
> > turn on the flags that correspond to features that are not implemented.
> > The problem here is that Hyper-V is about to introduce a feature without
> > a flag that is not implemented by Xen (see "X86: Deliver Hyper-V
> > interrupts on a separate IDT vector").
> > K.Y. please confirm if I got this right.
> 
> I am not sure I can agree with you here. There are two discriminating factors
> here: (a) Hypervisor check and (b) Feature check. Not every feature of the
> hypervisor can be surfaced as feature bit and furthermore, just because a
> feature
> bit is turned on, it does not necessarily mean that the feature is to be used. For
> instance,
> let us say that Windows guests begin to use the "partition counter" and Xen
> chooses
> to implement that to better support Windows. This does not mean that while
> hosting
> Linux on Xen, you want to plug in a clock source based on the emulated
> "partition counter". This is what would happen in the code we have today.
> 
> Other Hypervisors emulating Hyper-V do not have this problem and Xen would
> not either
> if the emulation bit is selectively turned on (only while running Windows) or if
> Xen were allowed
> to check first ahead of Hyper-V (unconditionally) in the hypervisor detection
> code. As Peter pointed out, we
> have this problem because of the unique situation with Xen.
> 
> In any event, I am not going to further argue this issue; this last round of patches I
> sent out,
> fixes the issue for Xen. Jan wants me to make this check more general. While I
> don't think
> we need to do that, I will see if I can do it. I am checking to see if MSFT
> guarantees that Hyper-V
> would initialize the unused CPUID space to 0. If this is the case, I will implement
> the check
> Jan has suggested; if not, we have to live with the Xen specific check that I
> currently have.

I checked with the Hyper-V guys and I am told that there is no guarantee that Hyper-V
would not use some other range in the CPUID space in the future. So, I will keep the Xen
specific check that I had in this version. I will add the appropriate compilation switches to
take care of the "dead code" and resubmit the patches.

Regards,


K. Y
> 
> >
> > If I were the Microsoft engineer implementing this feature, no matter
> > what Xen does or does not, I would also make sure that there is a
> > corresponding flag for it, because in my experience they avoid future
> > headaches.
> > I wonder what happens if you run Linux with Hyper-V support on an old
> > Hyper-V host that doesn't support vector injection.
> >
> 
> To answer your specific question, this feature of being able to distribute vmbus
> interrupt load across all VCPUs in the guest is a win8 and beyond feature. On prior
> hosts, all interrupts will be delivered on the boot CPU. VMBUS, as part of
> connecting with
> the hosts determines host supported protocol version and decides how it wants
> to
> program the hypervisor with regards to interrupt delivery. Even though we might
> setup
> an IDT entry for delivering the hypervisor interrupt, if the host is a pre-win8 host,
> the vmbus
> driver will program the hypervisor to deliver the interrupt on the boot CPU via a
> legacy interrupt
> vector.
> 
> Regards,
> 
> K. Y
> 
> _______________________________________________
> devel mailing list
> devel@linuxdriverproject.org
> http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
>