Cc Peterz, 2017-03-10 9:12 GMT+08:00 Michael S. Tsirkin <mst@xxxxxxxxxx>: > On Thu, Mar 09, 2017 at 07:51:27PM -0500, Gabriel L. Somlo wrote: >> On Fri, Mar 10, 2017 at 12:29:31AM +0200, Michael S. Tsirkin wrote: >> > 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. >> >> Are you getting an IPI if another VCPU writes to the MONITOR-ed memory >> location? > > In my testing yes. Why there is still an IPI if monitor/mwait is used in guest? > >> If not, you'd be waking up too late and fail to meet the >> specified behavior of the MONITOR/MWAIT instruction pair. >> >> > 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. 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. >> >> Is it a good idea to advertise MWAIT capability to guests? > > I think it isn't so this patch does not do it. > >> The >> misbehaving ones will call it willy-nilly, true, but aren't compliant >> ones better off falling back to some alternative method (typically >> using a HLT-based idle loop instead of a MONITOR/MWAIT based one) ? >> >> Thanks, >> --Gabriel >> [...] >> > @@ -594,6 +594,9 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function, >> > if (sched_info_on()) >> > entry->eax |= (1 << KVM_FEATURE_STEAL_TIME); >> > >> > + if (this_cpu_has(X86_FEATURE_MWAIT)) >> > + entry->eax = (1 << KVM_FEATURE_MWAIT); s/"="/"|=", otherwise you almost kill other features. Regards, Wanpeng Li
