Re: [PATCH] kvm: better MWAIT emulation for guests

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Mar 13, 2017 at 08:39:11PM +0100, Radim Krčmář wrote:
> 2017-03-13 18:08+0200, Michael S. Tsirkin:
> > On Mon, Mar 13, 2017 at 04:46:20PM +0100, Radim Krčmář wrote:
> >> 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?
> > 
> > If you are going into a deep sleep state with huge latency you are
> > better off exiting and paying an extra microsecond latency
> > since a chance some other task will want to schedule seems higher.
> 
> Oh, so MWAIT behavior is same and can reach deep sleep, just use-cases
> differ ... If the guest VCPU is running on isolated CPU, then you might
> want to reach a deep state to save power when there is no better use.
> 
> >> >                                                           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.
> > 
> > Current linux guests just discover these states based on
> > CPU model, so we do expose enough info.
> 
> Linux still filters the hardcoded hints through CPUID[5].edx, which is 0
> in our case.
> 
> >>    The feature would need very constrained setup, which is hard to
> >>    support
> > 
> > Why would it? It works without any tweaking on several boxes
> > I own.
> 
> MWAIT hints do not always mean the same, so they could lead to different
> power/performance tradeoffs than the applications expects.  We should at
> least specify that the paravirt feature allows only hint 0.
> 
> You probably don't run weird combinations of host/guest CPUs.
> 
> >>  - 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.
> > 
> > Yes I think this limited emulation is safe to enable by default.
> > Pretending mwait is equivalent to halt maybe isn't.
> 
> Right, we must keep the VCPU thread running when emulating mwait as it
> is different from a hlt.
> 
> >> What about keeping just the last hunk to improve OS X, for now?
> >> 
> >> Thanks.
> > 
> > IMHO if we have a new functionality we are better of creating
> > some way for guests to discover it is there.
> > 
> > Do we really have to argue about a single bit in HV leaf?
> > What harm does it do?
> 
> It adds code to both guest and hosts and needs documentation ...
> The bit is acceptable.  I just see no point in having it when there
> already is a detection mechanism for mwait.

We don't want to use that standard detection mechanism IMHO at least
not in all cases.

> In any case, this patch should also remove VM exits under SVM

AMD does not have MWAIT AFAIK. In any case, I don't see
why can't SVM be a separate patch.

> and add
> KVM_CAP_MWAIT for userspace.

Sure, why not. Will do.

> Userspace can then set the MWAIT feature
> if it wishes the guest to use it in a more standard way.
> 
> I can do a cleanup due to unused VM exits on top of it.
> 
> Thanks.