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

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

Hi Alex,

On Mon, Jul 25, 2022 at 11:06:24AM +0100, Alexandru Elisei wrote:

[...]

> > A funkier approach might be to defer pinning of the buffer until the SPE is
> > enabled and avoid pinning all of VM memory that way, although I can't
> > immediately tell how flexible the architecture is in allowing you to cache
> > the base/limit values.
> 
> I was investigating this approach, and Mark raised a concern that I think
> might be a showstopper.
> 
> Let's consider this scenario:
> 
> Initial conditions: guest at EL1, profiling disabled (PMBLIMITR_EL1.E = 0,
> PMBSR_EL1.S = 0, PMSCR_EL1.{E0SPE,E1SPE} = {0,0}).
> 
> 1. Guest programs the buffer and enables it (PMBLIMITR_EL1.E = 1).
> 2. Guest programs SPE to enable profiling at **EL0**
> (PMSCR_EL1.{E0SPE,E1SPE} = {1,0}).
> 3. Guest changes the translation table entries for the buffer. The
> architecture allows this.
> 4. Guest does an ERET to EL0, thus enabling profiling.
> 
> Since KVM cannot trap the ERET to EL0, it will be impossible for KVM to pin
> the buffer at stage 2 when profiling gets enabled at EL0.

Not saying we necessarily should, but this is possible with FGT no?

> I can see two solutions here:
> 
> a. Accept the limitation (and advertise it in the documentation) that if
> someone wants to use SPE when running as a Linux guest, the kernel used by
> the guest must not change the buffer translation table entries after the
> buffer has been enabled (PMBLIMITR_EL1.E = 1). Linux already does that, so
> running a Linux guest should not be a problem. I don't know how other OSes
> do it (but I can find out). We could also phrase it that the buffer
> translation table entries can be changed after enabling the buffer, but
> only if profiling happens at EL1. But that sounds very arbitrary.
> 
> b. Pin the buffer after the stage 2 DABT that SPE will report in the
> situation above. This means that there is a blackout window, but will
> happen only once after each time the guest reprograms the buffer. I don't
> know if this is acceptable. We could say that this if this blackout window
> is not acceptable, then the guest kernel shouldn't change the translation
> table entries after enabling the buffer.
> 
> Or drop the approach of pinning the buffer and go back to pinning the
> entire memory of the VM.
> 
> Any thoughts on this? I would very much prefer to try to pin only the
> buffer.

Doesn't pinning the buffer also imply pinning the stage 1 tables
responsible for its translation as well? I agree that pinning the buffer
is likely the best way forward as pinning the whole of guest memory is
entirely impractical.

I'm also a bit confused on how we would manage to un-pin memory on the
way out with this. The guest is free to muck with the stage 1 and could
cause the SPU to spew a bunch of stage 2 aborts if it wanted to be
annoying. One way to tackle it would be to only allow a single
root-to-target walk to be pinned by a vCPU at a time. Any time a new
stage 2 abort comes from the SPU, we un-pin the old walk and pin the new
one instead.

Live migration also throws a wrench in this. IOW, there are still potential
sources of blackout unattributable to guest manipulation of the SPU.

Going to think on this some more..

--
Thanks,
Oliver

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