2017-03-16 4:13 GMT+08:00 Radim Krčmář <rkrcmar@xxxxxxxxxx>:
> 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
How can guest rewrite this?
Regards,
Wanpeng Li
> 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.
