Re: [Qemu-devel] [PATCH] i386/kvm: add support for KVM_CAP_X86_DISABLE_EXITS

[Eduardo Habkost]

On Tue, Mar 27, 2018 at 10:42:56PM +0300, Michael S. Tsirkin wrote:
> On Fri, Mar 16, 2018 at 07:36:42AM -0700, Wanpeng Li wrote:
> > From: Wanpeng Li <address@hidden>
> > 
> > This patch adds support for KVM_CAP_X86_DISABLE_EXITS. Provides userspace 
> > with 
> > per-VM capability(KVM_CAP_X86_DISABLE_EXITS) to not intercept 
> > MWAIT/HLT/PAUSE 
> > in order that to improve latency in some workloads.
> > 
[...]
> > diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> > index d23fff1..95ed9eb 100644
> > --- a/target/i386/kvm.c
> > +++ b/target/i386/kvm.c
> > @@ -999,6 +999,18 @@ int kvm_arch_init_vcpu(CPUState *cs)
> >          }
> >      }
> >  
> > +    if (env->features[FEAT_KVM_HINTS] & KVM_HINTS_DEDICATED) {
> > +        int disable_exits = kvm_check_extension(cs->kvm_state, 
> > KVM_CAP_X86_DISABLE_EXITS);
> > +        if (disable_exits) {
> > +            disable_exits &= (KVM_X86_DISABLE_EXITS_MWAIT |
> > +                              KVM_X86_DISABLE_EXITS_HLT |
> > +                              KVM_X86_DISABLE_EXITS_PAUSE);
> > +        }
> > +        if (kvm_vm_enable_cap(cs->kvm_state, KVM_CAP_X86_DISABLE_EXITS, 0, 
> > disable_exits)) {
> > +            error_report("kvm: DISABLE EXITS not supported");
> > +        }
> > +    }
> > +
> >      qemu_add_vm_change_state_handler(cpu_update_state, env);
> >  
> >      c = cpuid_find_entry(&cpuid_data.cpuid, 1, 0);
> 
> Why not a bit per capability?
> I can see how someone might want to disable mwait exists
> but not the rest of them.

kvm-hint-dedicated=on should be used only if the physical CPU is
dedicated to the VCPU task.  Are there any advantages of getting
vmexits for HLT and PAUSE if no other task is going to use the
CPU?

-- 
Eduardo