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    SubjectRe: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V
    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
    > > >
    > > > --
    > > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    > > > the body of a message to majordomo@vger.kernel.org
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    > > > Please read the FAQ at http://www.tux.org/lkml/
    > >
    >


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