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

From: Radim KrÄmÃÅ
Date: Mon Mar 13 2017 - 15:39:41 EST

2017-03-13 18:08+0200, Michael S. Tsirkin:
> On Mon, Mar 13, 2017 at 04:46:20PM +0100, Radim KrÄmÃÅ 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.
>> 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@xxxxxxxxxx>
>> > ---
>> [...]
>> > 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.

In any case, this patch should also remove VM exits under SVM and add
KVM_CAP_MWAIT for userspace. 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.