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

[KY Srinivasan]

> -----Original Message-----
> From: Michael S. Tsirkin [mailto:mst at redhat.com]
> Sent: Wednesday, April 17, 2013 3:06 AM
> To: KY Srinivasan
> Cc: x86 at kernel.org; gregkh at linuxfoundation.org; linux-kernel at vger.kernel.org;
> devel at linuxdriverproject.org; olaf at aepfle.de; apw at canonical.com;
> jasowang at redhat.com; tglx at linutronix.de; hpa at zytor.com;
> JBeulich at suse.com; bp at 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 at 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. 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. 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
> 
> > ---
> >  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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