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

[Oliver Upton <oliver.upton@linux.dev>]

On Tue, Sep 13, 2022 at 01:41:56PM +0100, Alexandru Elisei wrote:
> 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).

Yup, that's it. You also get two whole bytes of room in PMBSR_EL1.MSS
which is also IMP DEF, so we could even stick some ASCII in there to
tell the guest how we really feel! :-P

> 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.

I agree with you that PMBSR_EL1.DL=1 is the right call for this. With
that, I'd be surprised if there was a guest that tried to pull some
tricks other than blowing away the profile. The other option that I
find funny is if we plainly report the S2 abort to the guest, but that
wont work well when nested comes into the picture.

--
Thanks,
Oliver

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