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

[Suzuki K Poulose]

Hi,

A few cents from the Realm support point of view.

On 05/02/2023 10:12, Marc Zyngier wrote:
> On Fri, 03 Feb 2023 13:50:40 +0000,
> James Morse <james.morse@xxxxxxx> wrote:
> > From: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> >
> > 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@xxxxxxx>
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> > Signed-off-by: James Morse <james.morse@xxxxxxx>
> >
> > ---
>
> 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.

This may not be always possible, e.g., for Realms. GET_ONE_REG is
not supported. So using an explicit passing down of the args is
preferrable.

Thanks
Suzuki