Re: KVM/arm64: SPE: Translate VA to IPA on a stage 2 fault instead of pinning VM memory

Alexandru Elisei
2022-09-13T12:42:16Z

Hi Oliver,

On Tue, Sep 13, 2022 at 11:58:47AM +0100, Oliver Upton wrote:
> Hey Alex,
>
> On Mon, Sep 12, 2022 at 03:50:46PM +0100, Alexandru Elisei wrote:
>
> [...]
>
> > > Yeah, that would be good to follow up on what other OSes are doing.
> >
> > FreeBSD doesn't have a SPE driver.
> >
> > Currently in the process of finding out how/if Windows implements the
> > driver.
> >
> > > You'll still have a nondestructive S2 fault handler for the SPE, right?
> > > IOW, if PMBSR_EL1.DL=0 KVM will just unpin the old buffer and repin the
> > > new one.
> >
> > This is how I think about it: a S2 DABT where DL == 0 can happen because of
> > something that the VMM, KVM or the guest has done:
> >
> > 1. If it's because of something that the host's userspace did (memslot was
> > changed while the VM was running, memory was munmap'ed, etc). In this case,
> > there's no way for KVM to handle the SPE fault, so I would say that the
> > sensible approach would be to inject an SPE external abort.
> >
> > 2. If it's because of something that KVM did, that can only be because of a
> > bug in SPE emulation. In this case, it can happen again, which means
> > arbitrary blackout windows which can skew the profiling results. I would
> > much rather inject an SPE external abort then let the guest rely on
> > potentially bad profiling information.
> >
> > 3. The guest changes the mapping for the buffer when it shouldn't have: A.
> > when the architecture does allow it, but KVM doesn't support, or B. when
> > the architecture doesn't allow it. For both cases, I would much rather
> > inject an SPE external abort for the reasons above. Furthermore, for B, I
> > think it would be better to let the guest know as soon as possible that
> > it's not following the architecture.
> >
> > In conclusion, I would prefer to treat all SPE S2 faults as errors.
>
> My main concern with treating S2 faults as a synthetic external abort is
> how this behavior progresses in later versions of the architecture.
> SPEv1p3 disallows implementations from reporting external aborts via the
> SPU, instead allowing only for an SError to be delivered to the core.

Ah, yes, missed that bit for SPEv1p3 (ARM DDI 0487H.a, page D10-5180).

>
> I caught up with Will on this for a little bit:
>
> Instead of an external abort, how about reporting an IMP DEF buffer
> management event to the guest? At least for the Linux driver it should
> have the same effect of killing the session but the VM will stay
> running. This way there's no architectural requirement to promote to an
> SError.

The only reason I proposed to inject an external abort is because KVM needs
a way to tell the guest that something outside of the guest's control went
wrong and it should drop the contents of the current profiling session. An
external abort reported by the SPU seemed to fit the bit.

By IMP DEF buffer management event I assume you mean PMBSR_EL1.EC=0b011111
(Buffer management event for an IMPLEMENTATION DEFINED reason). I'm
thinking that someone might run a custom kernel in a VM, like a vendor
downstream kernel, with patches that actually handle this exception class,
and injecting such an exception might not have the effects that KVM
expects. Am I overthinking things? Is that something that KVM should take
into consideration? I suppose KVM can and should also set
PMBSR_EL1.DL = 1, as that means per the architecture that the buffer
contents should be discarded.