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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 17, 2013 at 01:20:58PM +0000, KY Srinivasan wrote:
> 
> 
> > -----Original Message-----
> > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > Sent: Wednesday, April 17, 2013 3:06 AM
> > To: KY Srinivasan
> > Cc: x86@kernel.org; gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> > devel@linuxdriverproject.org; olaf@aepfle.de; apw@canonical.com;
> > jasowang@redhat.com; tglx@linutronix.de; hpa@zytor.com;
> > JBeulich@suse.com; bp@alien8.de
> > Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
> > 
> > On Sun, Feb 03, 2013 at 05:22:38PM -0800, K. Y. Srinivasan wrote:
> > > Xen emulates Hyper-V to host enlightened Windows. Looks like this
> > > emulation may be turned on by default even for Linux guests. Check and
> > > fail Hyper-V detection if we are on Xen.
> > >
> > > Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
> > 
> > I'm very curious what's the specific bug that is fixed here?
> > I went over the original discussion in
> > https://patchwork.kernel.org/patch/2064331/
> > and that's still not clear to me. Is there a configuration
> > that is broken without this patch but starts working with
> > this patch?
> > 
> > It seems that one might want to use hyper-v emulation e.g. to test
> > hyper-v code without using windows, so the functionality
> > that this patch disables is not completely useless,
> > so there should be a good reason for disabling it.
> > 
> > Could you enlighten me please? :)
> 
> Michael,
> 
> If I recall correctly, the issue here was that Xen was enabling
> Hyper-V emulation un-conditionally even for Linux guests.

Okay so basically some Xen specific strangeness.

> Clearly,
> this was not the intention to begin with. This check was added to
> ensure that while hosting Linux we would not un-intentionally enable
> Hyper-V emulation in Xen.

What if you want to intentionally enable Hyper-V emulation in Xen?
There's no way to do this now, right?

> Not all Hyper-V features are tagged with a
> CPUID feature bit and so it was agreed that it would be better to
> disable Hyper-V emulation when hosting Linux on Xen.
> 
> Regards,
> 
> K. Y

I was just asking whether there are any specific examples of a bug
or is this change done just in case?

> > 
> > > ---
> > >  arch/x86/kernel/cpu/mshyperv.c |    7 +++++++
> > >  1 files changed, 7 insertions(+), 0 deletions(-)
> > >
> > > diff --git a/arch/x86/kernel/cpu/mshyperv.c
> > b/arch/x86/kernel/cpu/mshyperv.c
> > > index 646d192..4dab317 100644
> > > --- a/arch/x86/kernel/cpu/mshyperv.c
> > > +++ b/arch/x86/kernel/cpu/mshyperv.c
> > > @@ -30,6 +30,13 @@ static bool __init ms_hyperv_platform(void)
> > >  	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
> > >  		return false;
> > >
> > > +	/*
> > > +	 * Xen emulates Hyper-V to support enlightened Windows.
> > > +	 * Check to see first if we are on a Xen Hypervisor.
> > > +	 */
> > > +	if (xen_cpuid_base())
> > > +		return false;
> > > +
> > >  	cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
> > >  	      &eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);
> > >
> > > --
> > > 1.7.4.1
> > >
> > > --
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>