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" <gso...@gmail.com>
> Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
> ---
>
> 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 <kernel...@gmail.com>
> - 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.
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