2017-03-15 21:28+0200, Michael S. Tsirkin: > Guests running Mac OS 5, 6, and 7 (Leopard through Lion) have a problem: > unless explicitly provided with kernel command line argument > "idlehalt=0" they'd implicitly assume MONITOR and MWAIT availability, > without checking CPUID. > > We currently emulate that as a NOP but on VMX we can do better: let > guest stop the CPU until timer, IPI or memory change. CPU will be busy > but that isn't any worse than a NOP emulation. > > Note that mwait within guests is not the same as on real hardware > because halt causes an exit while mwait doesn't. For this reason it > might not be a good idea to use the regular MWAIT flag in CPUID to > signal this capability. Add a flag in the hypervisor leaf instead. > > Additionally, we add a capability for QEMU - e.g. if it knows there's an > isolated CPU dedicated for the VCPU it can set the standard MWAIT flag > to improve guest behaviour. > > Reported-by: "Gabriel L. Somlo" <gsomlo@xxxxxxxxx> > Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx> > --- > > Note: SVM bits are untested at this point. Seems pretty > obvious though. > > changes from v3: > - don't enable capability if cli+mwait blocks interrupts > - doc typo fixes (drop drop ppc doc) > > changes from v2: > - add a capability to allow host userspace to detect new kernels > - more documentation to clarify the semantics of the feature flag > and why it's useful > - svm support as suggested by Radim > > changes from v1: > - typo fix resulting in rest of leaf flags being overwritten > Reported by: Wanpeng Li <kernellwp@xxxxxxxxx> > - updated commit log with data about guests helped by this feature > - better document differences between mwait and halt for guests > > diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h > @@ -212,4 +213,28 @@ static inline u64 nsec_to_cycles(struct kvm_vcpu *vcpu, u64 nsec) > __rem; \ > }) > > +static bool kvm_mwait_in_guest(void) > +{ > + unsigned int eax, ebx, ecx; > + > + if (!cpu_has(&boot_cpu_data, X86_FEATURE_MWAIT)) > + return -ENODEV; > + > + if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) > + return -ENODEV; > + > + /* > + * Intel CPUs without CPUID5_ECX_INTERRUPT_BREAK are problematic as > + * they would allow guest to stop the CPU completely by disabling > + * interrupts then invoking MWAIT. > + */ > + if (boot_cpu_data.cpuid_level < CPUID_MWAIT_LEAF) > + return -ENODEV; > + > + cpuid(CPUID_MWAIT_LEAF, &eax, &ebx, &ecx, &mwait_substates); > + > + if (!(ecx & CPUID5_ECX_INTERRUPT_BREAK)) > + return -ENODEV; The guest is still able to set ecx=0 with MWAIT, which should be the same as not having the CPUID flag, so I'm wondering how this check prevents anything harmful ... is it really a cpu "feature"? If we somehow report ecx bit 1 in CPUID[5], then the guest might try to set ecx bit 0 for MWAIT, which will cause #GP(0) and could explain the hang that Gabriel is hitting. Gabriel, - do you see VM exits on the "hung" VCPU? - what is your CPU model? - what do you get after running this C program on host and guest? #include <stdint.h> #include <stdio.h> int main(void) { uint32_t eax = 5, ebx, ecx = 0, edx; asm ("cpuid" : "+a"(eax), "=b"(ebx), "+c"(ecx), "=d"(edx)); printf("eax=%#08x ebx=%#08x ecx=%#08x edx=%#08x\n", eax, ebx, ecx, edx); return 0; } Thanks. -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html