Re: [RFC PATCH 0/5] KVM: arm64: Pass PSCI to userspace

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

Hi Alex,

I'm not planning to resend this work at the moment, because it looks like
vcpu hot-add will go a different way so I don't have a user. But I'll
probably address the feedback so far and park it on some branch, in case
anyone else needs it.

On Mon, Jul 19, 2021 at 04:29:18PM +0100, Alexandru Elisei wrote:
> 1. Why forwarding PSCI calls to userspace depend on enabling forwarding for other
> HVC calls? As I understand from the patches, those handle distinct function IDs.

The HVC cap from patch 4 enables returning from the VCPU_RUN ioctl with
KVM_EXIT_HYPERCALL, for any HVC not handled by KVM. This one should
definitely be improved, either by letting userspace choose the ranges of
HVC it wants, or at least by reporting ranges reserved by KVM to
userspace.

The PSCI cap from patch 5 disables the in-kernel PSCI implementation. As a
result those HVCs are forwarded to userspace.

It was suggested that other users will want to handle HVC calls (SDEI for
example [1]), hence splitting into two capabilities rather than just the
PSCI cap. In v5.14 x86 added KVM_CAP_EXIT_HYPERCALL [2], which lets
userspace receive specific hypercalls. We could reuse that and have PSCI
be one bit of that capability's parameter.

[1] https://lore.kernel.org/linux-arm-kernel/20170808164616.25949-12-james.morse@arm.com/
[2] https://lore.kernel.org/kvm/90778988e1ee01926ff9cac447aacb745f954c8c.1623174621.git.ashish.kalra@amd.com/

> 2. HVC call forwarding to userspace also forwards PSCI functions which are defined
> in ARM DEN 0022D, but not (yet) implemented by KVM. What happens if KVM's PSCI
> implementation gets support for one of those functions? How does userspace know
> that now it also needs to enable PSCI call forwarding to be able to handle that
> function?

We forward the whole PSCI function range, so it's either KVM or userspace.
If KVM manages PSCI and the guest calls an unimplemented function, that
returns directly to the guest without going to userspace.

The concern is valid for any other range, though. If userspace enables the
HVC cap it receives function calls that at some point KVM might need to
handle itself. So we need some negotiation between user and KVM about the
specific HVC ranges that userspace can and will handle.

> It looks to me like the boundary between the functions that are forwarded when HVC
> call forwarding is enabled and the functions that are forwarded when PSCI call
> forwarding is enabled is based on what Linux v5.13 handles. Have you considered
> choosing this boundary based on something less arbitrary, like the function types
> specified in ARM DEN 0028C, table 2-1?

For PSCI I've used the range 0-0x1f as the boundary, which is reserved for
PSCI by SMCCC (table 6-4 in that document).

> 
> In my opinion, setting the MP state to HALTED looks like a sensible approach to
> implementing PSCI_SUSPEND. I'll take a closer look at the patches after I get a
> better understanding about what is going on.
> 
> On 6/8/21 4:48 PM, Jean-Philippe Brucker wrote:
> > Allow userspace to request handling PSCI calls from guests. Our goal is
> > to enable a vCPU hot-add solution for Arm where the VMM presents
> > possible resources to the guest at boot, and controls which vCPUs can be
> > brought up by allowing or denying PSCI CPU_ON calls. Passing HVC and
> > PSCI to userspace has been discussed on the list in the context of vCPU
> > hot-add [1,2] but it can also be useful for implementing other SMCCC and
> > vendor hypercalls [3,4,5].
> >
> > Patches 1-3 allow userspace to request WFI to be executed in KVM. That
> 
> I don't understand this. KVM, in kvm_vcpu_block(), does not execute an WFI.
> PSCI_SUSPEND is documented as being indistinguishable from an WFI from the guest's
> point of view, but it's implementation is not architecturally defined.

Yes that was an oversimplification on my part

Thanks,
Jean