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

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

On Tue, Mar 27, 2018 at 06:36:46PM -0300, Eduardo Habkost wrote:
> 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 <wanpengli@tencent.com>
> > > 
> > > 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?

No but there are advantages to using mwait even without a dedicated host
CPU (VCPUs can wake up each other without exiting to hypervisor).

Which is my point - there should be a separate flag to disable mwait
exiting only.

> -- 
> Eduardo