> -----Original Message----- > From: Michael S. Tsirkin [mailto:mst@xxxxxxxxxx] > Sent: Wednesday, April 17, 2013 9:25 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 02:12:12PM +0000, KY Srinivasan wrote: > > > > > > > -----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). > > No, it's a Xen specific bug. KVM in a hyper-v mode still emulates PCI > so can still use KVM PV devices which are all PCI devices. So there's no problem > there. Even Xen does not have a problem here; Xen would install its PV drivers in the guest. So, the I/O infrastructure that would be installed in the guest would be based on the host. > > Also, why override this in the guest? With time Xen hyper-v emulation > might improve, meanwhile it's always possible to disable this in > the hypervisor. > > > 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 > > Why? Because there's no VMBUS on Xen? So check whether VMBUS exists? On Linux, Hyper-V detection is used to load hyper-V related drivers (VMBUS etc.). For the case in point, it was not clear to me how I could install the vmbus interrupt handler in a way that would make sense when Xen was unconditionally emulating Hyper-V. 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
