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 _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
