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

[Gabriel L. Somlo]

On Sun, Mar 12, 2017 at 02:01:32AM +0200, Michael S. Tsirkin wrote:
> On Fri, Mar 10, 2017 at 03:46:45PM -0800, Jim Mattson wrote:
> > On Thu, Mar 9, 2017 at 2:29 PM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > > Some guests call mwait without checking the cpu flags.  We currently
> > 
> > "Some guests"? What guests other than Mac OS X are so ill-behaved?
> 
> I heard about Mac OSX only but even that is hearsay for me
> so I didn't want to say that explicitly.

As the likely origin of said hearsay, I can confirm that Mac OS 5, 6,
and 7 (Leopard through Lion) had that problem: unless explicitly
provided with kernel command line argument "idlehalt=0" they'd
implicitly assume MONITOR and MWAIT availability, without checking CPUID.

As of MountainLion (10.8.x), that is no longer the case, and OS X
gracefully falls back to an idle loop which doesn't use MONITOR/MWAIT.

Regards,
--Gabriel
 
> > > 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.  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>
> > > ---
> > >  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