On Thu, Apr 09, 2015 at 01:06:47PM +0100, Andrew Jones wrote:
> On Thu, Apr 09, 2015 at 08:57:23AM +0100, Marc Zyngier wrote:
> > On Thu, 9 Apr 2015 02:46:54 +0100
> > Mario Smarduch <m.smarduch@xxxxxxxxxxx> wrote:
> >
> > Hi Mario,
> >
> > > I'm working with AsyncPF, and currently using
> > > hyp call to communicate guest GFN for host to inject
> > > virtual abort - page not available/page available.
> > >
> > > Currently only PSCI makes use of that interface,
> > > (handle_hvc()) can we overload interface with additional
> > > hyp calls in this case pass guest gfn? Set arg0
> > > to some range outside of PSCI use.
> >
> > I can't see a reason why we wouldn't open handle_hvc() to other
> > paravirtualized services. But this has to be done with extreme caution:
> >
> > - This becomes an ABI between host and guest
>
> To expand on that, if the benefits don't out weight the maintenance
> required for that ABI, for life, then it turns into a life-time burden.
> Any guest-host speedups that can be conceived, which require hypercalls,
> should probably be bounced of the hardware people first. Waiting for
> improvements in the virt extensions may be a better choice than
> committing to a PV solution.
>
> > - We need a discovery protocol
>
> Hopefully all users of the PSCI hypcall have been using function #0,
> because handle_hvc unfortunately hasn't been checking it. In any case,
> I'm not sure we have much choice but to start enforcing it now. Once we
> do, with something like
>
> switch(hypcall_nr) {
> case 0: /* handle psci call */
> default: return -KVM_ENOSYS;
> }
>
> then, I think the guest's discovery protocol can simply be
>
> if (do_hypercall() == -ENOSYS) {
> /* PV path not supported, fall back to whatever... */
> }
That only tells you the code at EL2/Hyp did something, and only if it
actually returns. Call this on a different hypervisor (or in the absence
of one, there's no mechanism for querying) and you might bring down that
CPU or the entire system.
We need to be able to detect that some hypercall interface is present
_before_ issuing the relevant hypercalls. As Marc mentioned, we could
have a DT node and/or ACPI entry for this, and it only needs to tell us
enough to bootstrap querying the hypervisor for more info (as is the
case with Xen, I believe).
>
> > - We need to make sure other hypervisors don't reuse the same function
> > number for other purposes
I don't think this is a problem so long as there's a mechanism for
detecting the hyp interfaces provided. Xen and KVM could use the same
numbers for different things and that's fine because you'll only use the
Xen functions when you see the Xen node, and the KVM functions when you
are aware you're under KVM. You can't have both simultaneously.
However, these numbers must be chosen so as not to clash with SMC/HVC
Calling Convention IDs. We can't risk clashing with PSCI or other
standard interfaces we may want to expose to a guest in future.
> I'm not sure what this means. Xen already has several hypercalls defined
> for ARM, the same that they have for x86, which don't match any of the
> KVM hypercalls. Now, KVM for other arches (which is maybe what you meant)
> does define a handful, which we should integrate with, as KVM mixes
> architectures within it's hypercall number allocation, see
> include/uapi/linux/kvm_para.h. Just using the common code should make it
> easy to avoid problems. We don't have a problem with the PSCI hypcall, as
> zero isn't allocated. Ideally we would define PSCI properly though,
> e.g. KVM_HC_ARM_PSCI, and still reserve zero in the common header. To do
> that maybe we'll need to keep #0 as an ARM-only alias for the new number
> for compatibility now?
While the HVC immediate could be used to distinguish different types of
calls, the guest still needs to first determine that issuing a HVC is
not going to bring down the system, which requires it to know that a
suitable hypervisor is present.
> > Maybe we should adopt Xen's idea of a hypervisor node in DT where we
> > would describe the various services? How will that work with ACPI?
>
> I don't think we'll ever have a "virt guest" ACPI table that we can
> use for this stuff, so this won't work for ACPI. But I think the ENOSYS
> probing should be sufficient for this anyway.
As mentioned above, I don't think that probing is safe.
What prevents us from creating a trivial "KVM guest" table that we can
use to determine that we can query more advanced info from KVM itself?
Given the point is to expose KVM-specific functionality, I don't see why
we need a more generic "virt guest" ACPI table.
Mark.
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