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

Oliver Upton
2022-08-17T15:07:31Z

Hi Alex,

On Fri, Aug 12, 2022 at 02:05:45PM +0100, Alexandru Elisei wrote:
> Hi Oliver,
>
> Just a note, for some reason some of your emails, but not all, don't show up in
> my email client (mutt). That's why it might take me a while to send a reply
> (noticed that you replied by looking for this thread on lore.kernel.org).

Urgh, that's weird. Am I getting thrown into spam or something? Also, do
you know if you've been receiving Drew's email since he switched to
@linux.dev?

> On Wed, Aug 10, 2022 at 10:25:56AM -0500, Oliver Upton wrote:
> > On Wed, Aug 10, 2022 at 10:37:26AM +0100, Alexandru Elisei wrote:
> > > Hi,
> > >
> > > On Tue, Aug 09, 2022 at 01:43:32PM -0500, Oliver Upton wrote:
> > > > Hi Alex,
> > > >
> > > > On Tue, Aug 09, 2022 at 03:01:36PM +0100, Alexandru Elisei wrote:
> > > >
> > > > [...]
> > > >
> > > > > > > To summarize the approaches we've discussed so far:
> > > > > > >
> > > > > > > 1. Pinning the entire guest memory
> > > > > > > - Heavy handed and not ideal.
> > > > > > > - Tried this approach in v5 of the SPE series [1], patches #2-#12.
> > > > > > >
> > > > > > > 2. Mapping the guest SPE buffer on demand, page by page, as a result of stage 2
> > > > > > > faults reported by SPE.
> > > > > > > - Not feasible, because the entire contents of the buffer must be discarded is
> > > > > > > PMBSR_EL1.DL is set to 1 when taking the fault.
> > > > > > > - Requires KVM to walk the guest's stage 1 tables, because SPE reports the VA,
> > > > > > > not the IPA.
> > > > > > >
> > > > > > > 3. Pinning the guest SPE buffer when profiling becomes enabled*:
> > > > > > > - There is the corner case described above, when profiling becomes enabled as a
> > > > > > > result of an ERET to EL0. This can happen when the buffer is enabled and
> > > > > > > PMSCR_EL1.{E0SPE,E1SPE} = {1,0};
> > > > > > > - The previous buffer is unpinned when a new buffer is pinned, to avoid SPE
> > > > > > > stage 2 faults when draining the buffer, which is performed with profiling
> > > > > > > disabled.
> > > > > > > - Also requires KVM to walk the guest's stage 1 tables.
> > > > > > >
> > > > > > > 4. Pin the entire guest SPE buffer after the first stage 2 fault reported by
> > > > > > > SPE.
> > > > > > > - Gets rid of the corner case at 3.
> > > > > > > - Same approach to buffer unpinning as 3.
> > > > > > > - Introduces a blackout window before the first record is written.
> > > > > > > - Also requires KVM to walk the guest's stage 1 tables.
> > > > > > >
> > > > > > > As for the corner case at 3, I proposed either:
> > > > > > >
> > > > > > > a) Mandate that guest operating systems must never modify the buffer
> > > > > > > translation entries if the buffer is enabled and
> > > > > > > PMSCR_EL1.{E0SPE,E1SPE} = {1,0}.
> > > > > > >
> > > > > > > b) Pin the entire buffer as a result of the first stage 2 fault reported by SPE,
> > > > > > > but **only** for this corner case. For all other cases, the buffer is pinned
> > > > > > > when profiling becomes enabled, to eliminate the blackout window. Guest
> > > > > > > operating systems can be modified to not change the translation entries for the
> > > > > > > buffer if this blackout window is not desirable.
> > > > > > >
> > > > > > > Pinning as a result of the **first** stage 2 fault should work, because there
> > > > > > > are no prior records that would have to be discarded if PMSBR_EL1.DL = 1.
> > > > > > >
> > > > > > > I hope I haven't missed anything. Thoughts and suggestions more than welcome.
> > > > > >
> > > > > > Thanks Alex for pulling together all of the context here.
> > > > > >
> > > > > > Unless there's any other strong opinions on the topic, it seems to me
> > > > > > that option #4 (pin on S2 fault) is probably the best approach for
> > > > > > the initial implementation. No amount of tricks in KVM can work around
> > > > > > the fact that SPE has some serious issues w.r.t. virtualization. With
> > > > > > that, we should probably document the behavior of SPE as a known erratum
> > > > > > of KVM.
> > > > > >
> > > > > > If folks complain about EL1 profile blackout, eagerly pinning when
> > > > > > profiling is enabled could layer on top quite easily by treating it as
> > > > > > a synthetic S2 fault and triggering the implementation of #4. Having
> > > > >
> > > > > I'm not sure I follow, I understand what you mean by "treating it as a
> > > > > synthetic S2 fault", would you mind elaborating?
> > > >
> > > > Assuming approach #4 is implemented, we will already have an SPE fault
> > > > handler that walks stage-1 and pins the buffer. At that point,
> > > > implementing approach #3 would be relatively easy. When EL1 sets
> > > > PMSCR_EL1.E1SPE, call the SPE fault handler on the GVA of the buffer.
> > >
> > > I see, that makes sense, thanks,
> > >
> > > >
> > > > > > said that I don't believe it is a hard requirement for enabling some
> > > > > > flavor of SPE for guests.
> > > > > >
> > > > > > Walking guest S1 in KVM doesn't sound too exciting although it'll need to
> > > > > > be done eventually.
> > > > > >
> > > > > > Do you feel like this is an OK route forward, or have I missed
> > > > > > something?
> > > > >
> > > > > I've been giving this some thought, and I prefer approach #3 because with
> > > > > #4, pinning the buffer as a result of a stage 2 fault reported by SPE, it
> > > > > will be impossible to distinguish between a valid stage 2 fault (a fault
> > > > > caused by the guest reprogramming the buffer and enabling profiling) and
> > > > > KVM messing something up when pinning the buffer. I believe this to be
> > > > > important, as experience has shown me that pinning the buffer at stage 2 is
> > > > > not trivial and there isn't a mechanism today in Linux to do that
> > > > > (explanation and examples here [1]).
> > > >
> > > > How does eagerly pinning avoid stage-2 aborts, though? As you note in
> > > > [1], page pinning does not avoid the possibility of the MMU notifiers
> > > > being called on a given range. Want to make sure I'm following, what
> > > > is your suggestion for approach #3 to handle the profile buffer when
> > > > only enabled at EL0?
> > > >
> > > > > With approach #4, it would be impossible to figure out if the results of a
> > > > > profiling operations inside a guest are representative of the workload or
> > > > > not, because those SPE stage 2 faults triggered by a bug in KVM can happen
> > > > > multiple times per profiling session, introducing multiple blackout windows
> > > > > that can skew the results.
> > > > >
> > > > > If you're proposing that the blackout window when the first record is
> > > > > written be documented as an erratum for KVM, then why no got a step further
> > > > > and document as an erratum that changing the buffer translation tables
> > > > > after the buffer has been enabled will lead to an SPE Serror? That will
> > > > > allow us to always pin the buffer when profiling is enabled.
> > > >
> > > > Ah, there are certainly more errata in virtualizing SPE beyond what I
> > > > had said :) Preserving the stage-1 translations while profiling is
> > > > active is a good recommendation, although I'm not sure that we've
> > > > completely eliminated the risk of stage-2 faults.
> > > >
> > > > It seems impossible to blame the guest for all stage-2 faults that happen
> > > > in the middle of a profiling session. In addition to host mm driven changes
> > > > to stage-2, live migration is a busted as well. You'd need to build out
> > > > stage-2 on the target before resuming the guest and guarantee that the
> > > > appropriate pages have been demanded from the source (in case of post-copy).
> > > >
> > > > So, are we going to inject an SError for stage-2 faults outside of guest
> > > > control as well? An external abort reported as an SPE buffer management
> > > > event seems to be gracefully handled by the Linux driver, but that behavior
> > > > is disallowed by SPEv1p3.
> > > >
> > > > To sum up the point I'm getting at: I agree that there are ways to
> > > > reduce the risk of stage-2 faults in the middle of profiling, but I
> > > > don't believe the current architecture allows KVM to virtualize the
> > > > feature to the letter of the specification.
> > >
> > > I believe there's some confusion here: emulating SPE **does not work** if
> > > stage 2 faults are triggered in the middle of a profiling session. Being
> > > able to have a memory range never unmapped from stage 2 is a
> > > **prerequisite** and is **required** for SPE emulation, it's not a nice to
> > > have.
> > >
> > > A stage 2 fault before the first record is written is acceptable because
> > > there are no other records already written which need to be thrown away.
> > > Stage 2 faults after at least one record has been written are unacceptable
> > > because it means that the contents of the buffer needs to thrown away.
> > >
> > > Does that make sense to you?
> > >
> > > I believe it is doable to have addresses always mapped at stage 2 with some
> > > changes to KVM, but that's not what this thread is about. This thread is
> > > about how and when to pin the buffer.
> >
> > Sorry if I've been forcing a tangent, but I believe there is a lot of
> > value in discussing what is to be done for keeping the stage-2 mapping
> > alive. I've been whining about it out of the very concern you highlight:
> > a stage-2 fault in the middle of the profile is game over. Otherwise,
> > optimizations in *when* we pin the buffer seem meaningless as stage-2
> > faults appear unavoidable.
>
> The idea I had was to propagate the mmu_notifier_range->event field to the
> arch code. Then keep track of the IPAs which KVM pinned with
> pin_user_page(s) that translate the guest buffer, and don't unmap that IPA
> from stage 2 if the event != MMU_NOTIFY_UNMAP. For a pinned page, all
> notifier events except MMU_NOTIFY_UNMAP are caused by the mm subsystem
> trying to change how that particular page is mapped.
>
> >
> > Nonetheless, back to your proposal. Injecting some context from earlier:
> >
> > > 3. Pinning the guest SPE buffer when profiling becomes enabled*:
> >
> > So we are only doing this when enabled for EL1, right?
> > (PMSCR_EL1.{E0SPE,E1SPE} = {x, 1})
>
> Yes, pin when PMBLIMITR_EL1.E = 1 and PMSCR_EL1.{E0SPE,E1SPE} = {x, 1}.
> Accesses to those registers can be trapped by KVM, and to verify the
> condition becomes trivial.
>
> >
> > > - There is the corner case described above, when profiling becomes enabled as a
> > > result of an ERET to EL0. This can happen when the buffer is enabled and
> > > PMSCR_EL1.{E0SPE,E1SPE} = {1,0};
> >
> > Is your proposal for the EL0 case to pin on fault or pin when E0SPE is set
> > (outside of the architectures definition of when profiling is enabled)?
>
> The original proposal was to pin on the first fault in this case, yes.
> That's because the architecture doesn't forbid changing the translation
> entries for the buffer when PMBLIMITR_EL1.E = 1 and sampling is disabled
> (PMSCR_EL1.{E0SPE, E1SPE] = {x, 0}).
>
> But you mentioned adding a quirk/erratum to KVM in your proposal, and I was
> thinking that we could add an erratum to avoid the case above by saying
> that that behaviour is impredictable. But that might restrict what
> operating systems KVM can run in an SPE-enabled VM, I can do some digging
> to find out how other operating systems use SPE, if you think adding the
> quirk sounds reasonable.

Yeah, that would be good to follow up on what other OSes are doing.
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.

--
Thanks,
Oliver