> -----Original Message----- > From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx] > Sent: Wednesday, April 17, 2013 8:49 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 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? Ideally, if Hyper-V could be emulated in a plug compatible way, in that the guest would not know the difference as to whether Hyper-V was being emulated or indeed it was Hyper-V underneath, I would agree with you. Even in this case, I would argue that we would somehow give preference to the native implementation rather than the emulated environment. Having done the initial emulation of Hyper-V on Xen, I know the current emulation of Hyper-V is restricted to emulating a few features advertised via the CPUID bits. If I recall correctly, Xen did not implement the enlightened TLB flush scheme. Furthermore, none of the Hyper-V emulations implement the I/O related infrastructure. For Linux to run efficiently on Hyper-V, the I/O paths are significantly more important than what Hyper-V emulation there is in Xen and I suspect in KVM. So, I don't think we want to run Linux on Xen/KVM that is emulating Hyper-V (cpuids). The specific case I ran into that required this patch was when I moved the VMBUS interrupts to be delivered via a special hypervisor vector (to support full interrupt distribution amongst all VCPUs in the guest). This feature is not tagged in the CPUID feature bits and without this check, I would install an interrupt handler for the hypervisor vector that clearly did not make any sense in anything other than Hyper-V. I suspect there will be more such situations as we go forward. 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 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
