Subject: [PATCH v3 1/4] ARM: KVM: Implement kvm_vcpu_preferred_target() function

From: [email-masked] (Christoffer Dall)
Date: Mon, 23 Sep 2013 16:31:06 +0100

On Mon, Sep 23, 2013 at 01:35:20PM +0530, Anup Patel wrote:
> Hi Christoffer/Marc,
> 
> On Fri, Sep 20, 2013 at 12:50 AM, Christoffer Dall
> <[email-masked]> wrote:
> > On Thu, Sep 19, 2013 at 03:27:54PM +0100, Marc Zyngier wrote:
> >> On 19/09/13 14:11, Anup Patel wrote:
> >> > This patch implements kvm_vcpu_preferred_target() function for
> >> > KVM ARM which will help us implement KVM_ARM_PREFERRED_TARGET ioctl
> >> > for user space.
> >> >
> >> > Signed-off-by: Anup Patel <[email-masked]>
> >> > Signed-off-by: Pranavkumar Sawargaonkar <[email-masked]>
> >> > ---
> >> >  arch/arm/kvm/guest.c              |   20 ++++++++++++++++++++
> >> >  arch/arm64/include/asm/kvm_host.h |    1 +
> >> >  2 files changed, 21 insertions(+)
> >> >
> >> > diff --git a/arch/arm/kvm/guest.c b/arch/arm/kvm/guest.c
> >> > index 152d036..b407e6c 100644
> >> > --- a/arch/arm/kvm/guest.c
> >> > +++ b/arch/arm/kvm/guest.c
> >> > @@ -222,6 +222,26 @@ int kvm_vcpu_set_target(struct kvm_vcpu *vcpu,
> >> >     return kvm_reset_vcpu(vcpu);
> >> >  }
> >> >
> >> > +int kvm_vcpu_preferred_target(struct kvm_vcpu_init *init)
> >> > +{
> >> > +   int target = kvm_target_cpu();
> >> > +
> >> > +   if (target < 0)
> >> > +           return -ENODEV;
> >> > +
> >> > +   memset(init, 0, sizeof(*init));
> >> > +
> >> > +   /*
> >> > +    * For now, we return all optional features are available
> >> > +    * for preferred target. In future, we might have features
> >> > +    * available based on underlying host.
> >> > +    */
> >> > +   init->target = (__u32)target;
> >> > +   init->features[0] |= (1 << KVM_ARM_VCPU_POWER_OFF);
> >>
> >> I'm in two minds about this feature reporting. I see they serve a
> >> purpose, but they also duplicate capabilities, which is the standard way
> >> to advertise what KVM can do.
> >>
> >> It means we end up having to sync two reporting mechanism, and I feel
> >> this is in general a bad idea.
> >>
> >> Furthermore, KVM_ARM_VCPU_POWER_OFF is hardly a feature of the HW, but
> >> rather a firmware emulation thing.
> >>
> >> Peter, Christoffer: Thoughts?
> >>
> > I wanted to return the full kvm_vcpu_init instead of just a target int,
> > so we did not have to come up with yet another ioctl if we need to
> > return more information about the capabilities of the host CPU in the
> > future.
> >
> > But perhaps we can formulate the API, to say only the (currently empty)
> > following list of features can only be enabled if the corresponding bit
> > is enabled, or something along those lines.
> >
> > -Christoffer
> > _______________________________________________
> > kvmarm mailing list
> > kvmarm at lists.cs.columbia.edu
> > https://lists.cs.columbia.edu/cucslists/listinfo/kvmarm
> 
> Do we stick with current implementation of returning struct kvm_vcpu_init ?
> OR
> Do we return struct kvm_vcpu_init with all features set to zero ?
> 
Let's give Marc a day or two to respond to this one ;)

-Christoffer