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SubjectRe: [PATCH] kvm: better MWAIT emulation for guests
2017-03-10 00:29+0200, Michael S. Tsirkin:
> Some guests call mwait without checking the cpu flags. We currently
> emulate that as a NOP but on VMX we can do better: let guest stop the
> CPU until timer or IPI. 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 you must halt if you want to go deep into sleep.

SDM (25.3 CHANGES TO INSTRUCTION BEHAVIOR IN VMX NON-ROOT OPERATION)
says that "MWAIT operates normally". What is the reason why MWAIT
inside VMX cannot reach the same states as MWAIT outside VMX?

> Thus it isn't
> a good idea to use the regular MWAIT flag in CPUID for that. Add a flag
> in the hypervisor leaf instead.
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
[...]
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> @@ -594,6 +594,9 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
> + if (this_cpu_has(X86_FEATURE_MWAIT))
> + entry->eax = (1 << KVM_FEATURE_MWAIT);

I'd rather not add it as a paravirt feature:

- MWAIT requires the software to provide a target state, but we're not
doing anything to expose those states.
The feature would need very constrained setup, which is hard to
support

- we've had requests to support MWAIT emulation for Linux and fully
emulating MWAIT would be best.
MWAIT is not going to enabled by default, of course; it would be
targeted at LPAR-like uses of KVM.

What about keeping just the last hunk to improve OS X, for now?

Thanks.

> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> @@ -3547,13 +3547,9 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf)
> CPU_BASED_USE_IO_BITMAPS |
> CPU_BASED_MOV_DR_EXITING |
> CPU_BASED_USE_TSC_OFFSETING |
> - CPU_BASED_MWAIT_EXITING |
> - CPU_BASED_MONITOR_EXITING |
> CPU_BASED_INVLPG_EXITING |
> CPU_BASED_RDPMC_EXITING;
>
> - printk(KERN_ERR "cleared CPU_BASED_MWAIT_EXITING + CPU_BASED_MONITOR_EXITING\n");
> -
> opt = CPU_BASED_TPR_SHADOW |
> CPU_BASED_USE_MSR_BITMAPS |
> CPU_BASED_ACTIVATE_SECONDARY_CONTROLS;
> --
> MST

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