Re: [RFC PATCH 29/32] KVM: arm64: Pass hypercalls to userspace

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

On Sun, Feb 05, 2023 at 10:12:49AM +0000, Marc Zyngier wrote:
> On Fri, 03 Feb 2023 13:50:40 +0000,
> James Morse <james.morse@arm.com> wrote:
> > 
> > From: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > 
> > When capability KVM_CAP_ARM_HVC_TO_USER is available, userspace can
> > request to handle all hypercalls that aren't handled by KVM. With the
> > help of another capability, this will allow userspace to handle PSCI
> > calls.
> > 
> > Suggested-by: James Morse <james.morse@arm.com>
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > Signed-off-by: James Morse <james.morse@arm.com>
> > 
> > ---
> > 
> 
> On top of Oliver's ask not to make this a blanket "steal everything",
> but instead to have an actual request for ranges of forwarded
> hypercalls:
> 
> > Notes on this implementation:
> > 
> > * A similar mechanism was proposed for SDEI some time ago [1]. This RFC
> >   generalizes the idea to all hypercalls, since that was suggested on
> >   the list [2, 3].
> > 
> > * We're reusing kvm_run.hypercall. I copied x0-x5 into
> >   kvm_run.hypercall.args[] to help userspace but I'm tempted to remove
> >   this, because:
> >   - Most user handlers will need to write results back into the
> >     registers (x0-x3 for SMCCC), so if we keep this shortcut we should
> >     go all the way and read them back on return to kernel.
> >   - QEMU doesn't care about this shortcut, it pulls all vcpu regs before
> >     handling the call.
> >   - SMCCC uses x0-x16 for parameters.
> >   x0 does contain the SMCCC function ID and may be useful for fast
> >   dispatch, we could keep that plus the immediate number.
> > 
> > * Add a flag in the kvm_run.hypercall telling whether this is HVC or
> >   SMC?  Can be added later in those bottom longmode and pad fields.
> 
> We definitely need this. A nested hypervisor can (and does) use SMCs
> as the conduit. The question is whether they represent two distinct
> namespaces or not. I *think* we can unify them, but someone should
> check and maybe get clarification from the owners of the SMCCC spec.
> 
> >
> > * On top of this we could share with userspace which HVC ranges are
> >   available and which ones are handled by KVM. That can actually be added
> >   independently, through a vCPU/VM device attribute which doesn't consume
> >   a new ioctl:
> >   - userspace issues HAS_ATTR ioctl on the vcpu fd to query whether this
> >     feature is available.
> >   - userspace queries the number N of HVC ranges using one GET_ATTR.
> >   - userspace passes an array of N ranges using another GET_ATTR. The
> >     array is filled and returned by KVM.
> 
> As mentioned above, I think this interface should go both ways.
> Userspace should request the forwarding of a certain range of
> hypercalls via a similar SET_ATTR interface.
> 
> Another question is how we migrate VMs that have these forwarding
> requirements. Do we expect the VMM to replay the forwarding as part of
> the setting up on the other side? Or do we save/restore this via a
> firmware pseudo-register?

Personally I'd prefer we left that job to userspace.

We could also implement GET_ATTR, in case userspace has forgotten what
it wrote to the hypercall filter. The firmware pseudo-registers are
handy for moving KVM state back and forth 'for free', but I don't think
we need to bend over backwards to migrate state userspace is directly
responsible for.

-- 
Thanks,
Oliver