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. > 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 > > > > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > > --- > > Documentation/virtual/kvm/cpuid.txt | 3 +++ > > arch/x86/include/uapi/asm/kvm_para.h | 1 + > > arch/x86/kvm/cpuid.c | 3 +++ > > arch/x86/kvm/vmx.c | 4 ---- > > 4 files changed, 7 insertions(+), 4 deletions(-) > > > > diff --git a/Documentation/virtual/kvm/cpuid.txt > > b/Documentation/virtual/kvm/cpuid.txt > > index 3c65feb..5caa234 100644 > > --- a/Documentation/virtual/kvm/cpuid.txt > > +++ b/Documentation/virtual/kvm/cpuid.txt > > @@ -54,6 +54,9 @@ KVM_FEATURE_PV_UNHALT || 7 || guest > > checks this feature bit > > || || before enabling > > paravirtualized > > || || spinlock support. > > > > ------------------------------------------------------------------------------ > > +KVM_FEATURE_MWAIT || 8 || guest can use monitor/mwait > > + || || to halt the VCPU. > > +------------------------------------------------------------------------------ > > KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no > > guest-side > > || || per-cpu warps are expected > > in > > || || kvmclock. > > diff --git a/arch/x86/include/uapi/asm/kvm_para.h > > b/arch/x86/include/uapi/asm/kvm_para.h > > index cff0bb6..9cc77a7 100644 > > --- a/arch/x86/include/uapi/asm/kvm_para.h > > +++ b/arch/x86/include/uapi/asm/kvm_para.h > > @@ -24,6 +24,7 @@ > > #define KVM_FEATURE_STEAL_TIME 5 > > #define KVM_FEATURE_PV_EOI 6 > > #define KVM_FEATURE_PV_UNHALT 7 > > +#define KVM_FEATURE_MWAIT 8 > > > > /* The last 8 bits are used to indicate how to interpret the flags field > > * in pvclock structure. If no bits are set, all flags are ignored. > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > > index efde6cc..fe3d292 100644 > > --- 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 (sched_info_on()) > > entry->eax |= (1 << KVM_FEATURE_STEAL_TIME); > > > > + if (this_cpu_has(X86_FEATURE_MWAIT)) > > + entry->eax = (1 << KVM_FEATURE_MWAIT); > > + > > entry->ebx = 0; > > entry->ecx = 0; > > entry->edx = 0; > > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c > > index 4bfe349..b167aba 100644 > > --- 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
