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

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

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 <mst@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