On Wed, Apr 17, 2013 at 01:20:58PM +0000, KY Srinivasan wrote: > > > > -----Original Message----- > > From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx] > > Sent: Wednesday, April 17, 2013 3:06 AM > > To: KY Srinivasan > > Cc: x86@xxxxxxxxxx; gregkh@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; > > devel@xxxxxxxxxxxxxxxxxxxxxx; olaf@xxxxxxxxx; apw@xxxxxxxxxxxxx; > > jasowang@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; hpa@xxxxxxxxx; > > JBeulich@xxxxxxxx; bp@xxxxxxxxx > > 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@xxxxxxxxxxxxx> > > > > 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@xxxxxxxxxxxxxxx > > > More majordomo info at http://vger.kernel.org/majordomo-info.html > > > Please read the FAQ at http://www.tux.org/lkml/ > > > _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
