RE: [RFC PATCH v2 2/3] KVM: x86: move ()_in_guest checking to vCPU scope

[Kechen Lu]

> -----Original Message-----
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Sent: Monday, January 10, 2022 11:36 AM
> To: Kechen Lu <kechenl@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx; pbonzini@xxxxxxxxxx; wanpengli@xxxxxxxxxxx;
> vkuznets@xxxxxxxxxx; mst@xxxxxxxxxx; Somdutta Roy
> <somduttar@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFC PATCH v2 2/3] KVM: x86: move ()_in_guest checking to
> vCPU scope
> 
> External email: Use caution opening links or attachments
> 
> 
> The shortlog is weird, I get what you're going for with the "()", but it honestly
> looks like a typo :-)  And add "power management" so that there's a bit more
> context in the shortlog?  Maybe this?
> 
>   KVM: x86: Move *_in_guest power management flags to vCPU scope
> 

Ack. Yeah it's a typo :)

> On Tue, Dec 21, 2021, Kechen Lu wrote:
> > For futher extensions on finer-grained control on per-vCPU exits
> > disable control, and because VM-scoped restricted to set before vCPUs
> > creation, runtime disabled exits flag check could be purely vCPU
> > scope.
> 
> State what the patch does, not what it "could" do.  E.g.
> 
> Make the runtime disabled mwait/hlt/pause/cstate exits flags vCPU scope to
> allow finer-grained, per-vCPU control.  The VM-scoped control is only
> allowed before vCPUs are created, thus preserving the existing behavior is a
> simple matter of snapshotting the flags at vCPU creation.
> 

Ack! Thanks for patient refinement on the description wording :P

> A few nits below that aren't even from this path, but otherwise,
> 
> Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> > Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > Signed-off-by: Kechen Lu <kechenl@xxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  5 +++++
> >  arch/x86/kvm/cpuid.c            |  2 +-
> >  arch/x86/kvm/lapic.c            |  2 +-
> >  arch/x86/kvm/svm/svm.c          | 10 +++++-----
> >  arch/x86/kvm/vmx/vmx.c          | 16 ++++++++--------
> >  arch/x86/kvm/x86.c              |  6 +++++-
> >  arch/x86/kvm/x86.h              | 16 ++++++++--------
> >  7 files changed, 33 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/include/asm/kvm_host.h
> > b/arch/x86/include/asm/kvm_host.h index 2164b9f4c7b0..edc5fca4d8c8
> > 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -908,6 +908,11 @@ struct kvm_vcpu_arch {  #if
> > IS_ENABLED(CONFIG_HYPERV)
> >       hpa_t hv_root_tdp;
> >  #endif
> > +
> > +     bool mwait_in_guest;
> > +     bool hlt_in_guest;
> > +     bool pause_in_guest;
> > +     bool cstate_in_guest;
> >  };
> >
> >  struct kvm_lpage_info {
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index
> > 07e9215e911d..6291e15710ba 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -177,7 +177,7 @@ void kvm_update_cpuid_runtime(struct kvm_vcpu
> *vcpu)
> >               best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> >
> >       best = kvm_find_kvm_cpuid_features(vcpu);
> > -     if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> > +     if (kvm_hlt_in_guest(vcpu) && best &&
> >               (best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
> 
> Can you (or Paolo?) opportunistically align this?  And maybe even shuffle the
> check on "best" to pair the !NULL check with the functional check?  E.g.
> 
>         if (kvm_hlt_in_guest(vcpu) &&
>             best && (best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
>                 best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> 

Makes sense. Let me align and reform this in this patch.

> >               best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c index
> > f206fc35deff..effb994e6642 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -119,7 +119,7 @@ static bool kvm_can_post_timer_interrupt(struct
> > kvm_vcpu *vcpu)  bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)  {
> >       return kvm_x86_ops.set_hv_timer
> > -            && !(kvm_mwait_in_guest(vcpu->kvm) ||
> > +            && !(kvm_mwait_in_guest(vcpu) ||
> 
> And another opportunistic tweak?  I'm been itching for an excuse to "fix" this
> particular helper for quite some time :-)
> 
>         return kvm_x86_ops.set_hv_timer &&
>                !(kvm_mwait_in_guest(vcpu) ||
> kvm_can_post_timer_interrupt(vcpu));
> 
> That overruns the 80 char soft limit, but IMO it's worth it in this case as the
> resulting code is more readable.
> 

Sure!

Best Regards,
Kechen

> 
> >                   kvm_can_post_timer_interrupt(vcpu));
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_can_use_hv_timer);