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

[Marc Zyngier <maz@kernel.org>]

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?

> 
> * Enabling this using a vCPU arch feature rather than the whole-VM
>   capability would be fine, but it would be difficult to do the same for
>   the following psci-in-user capability. So let's enable everything at
>   the VM scope.

Absolutely.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.