> -----Original Message----- > From: Michael S. Tsirkin [mailto:m...@redhat.com] > Sent: Wednesday, April 17, 2013 3:06 AM > To: KY Srinivasan > Cc: x...@kernel.org; gre...@linuxfoundation.org; linux-kernel@vger.kernel.org; > de...@linuxdriverproject.org; o...@aepfle.de; a...@canonical.com; > jasow...@redhat.com; t...@linutronix.de; h...@zytor.com; > jbeul...@suse.com; b...@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 <k...@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 > > > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > > the body of a message to majord...@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
