Re: [RFC PATCH 2/3] KVM: arm64: Allow userspace to trap SMCCC sub-ranges

[Oliver Upton <oliver.upton@linux.dev>]

On Fri, Nov 18, 2022 at 02:56:38PM +0000, Will Deacon wrote:
> On Thu, Nov 10, 2022 at 01:53:26AM +0000, Oliver Upton wrote:
> > As the SMCCC (and related specifications) march towards an
> > 'everything and the kitchen sink' interface for interacting with a
> > system, it is less likely that KVM will implement every supported
> > feature.
> > 
> > Add a capability that allows userspace to trap hypercall ranges,
> > allowing the VMM to mix-and-match between calls handled in userspace vs.
> > KVM.
> > 
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> > ---
> >  arch/arm64/include/asm/kvm_host.h |  5 ++++
> >  arch/arm64/include/uapi/asm/kvm.h | 15 ++++++++++
> >  arch/arm64/kvm/arm.c              | 10 +++++++
> >  arch/arm64/kvm/hypercalls.c       | 48 +++++++++++++++++++++++++++++++
> >  include/uapi/linux/kvm.h          |  1 +
> >  5 files changed, 79 insertions(+)
> 
> [...]
> 
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 6f0b56e7f8c7..6e8a222fc295 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -100,6 +100,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >  		r = 0;
> >  		set_bit(KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED, &kvm->arch.flags);
> >  		break;
> > +	case KVM_CAP_ARM_USER_HYPERCALLS:
> > +		if (cap->args[0] & ~KVM_ARM_USER_HYPERCALL_FLAGS)
> > +			return -EINVAL;
> 
> Why not use KVM_CAP_EXIT_HYPERCALL for this?

Err... I hilariously hijacked its UAPI for the exit but added a new cap
for it :)

I think the direction going forward will be to provide userspace with a
range-based filter such that (to a degree) we can arbitrarily forward
hypercalls to userspace, allowing for a mix-and-match approach.

> At some point during pKVM
> development, we used that to notify the VMM about memory being shared
> back from the guest but we eventually dropped it as the notification to
> userspace ended up not being needed:
> 
> https://android-kvm.googlesource.com/linux/+/dbd2861832dfc4c8a3103214b3c212ee7ace1c44%5E%21/
> https://android-kvm.googlesource.com/linux/+/2a3afc6da99c0e0cb62be1687153ee572903aa80%5E%21/
> 
> I'm not saying that what we did was necessarily better, but it seems a bit
> simpler and I figured it might be useful to point you to it.

Yeah, this is certainly a lot cleaner than what I've proposed here. And
frankly, for my immediate interest (forwarding vendor hypercalls to
userspace), this would fit the bill. OTOH, I was hoping that something
a bit more flexible could move the onus of implementing every darn spec
onto userspace (where possible).

I know you said pKVM has no need for userspace notifications at this
moment, but could user hypercalls be useful again going forward?

--
Thanks,
Oliver