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@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. > > - 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
