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@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. 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.
