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

Marc Zyngier
2022-07-27T11:09:22Z

On 2022-07-27 11:44, Alexandru Elisei wrote:
> On Wed, Jul 27, 2022 at 11:29:03AM +0100, Marc Zyngier wrote:
>> On 2022-07-27 11:19, Alexandru Elisei wrote:
>> > Hi Oliver,
>> >
>> > Thank you for the help, replies below.
>> >
>> > On Tue, Jul 26, 2022 at 10:51:21AM -0700, Oliver Upton wrote:
>> > > 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?
>> >
>> > It doesn't look to me like FEAT_FGT offers any knobs to trap ERET from
>> > EL1.
>>
>> See HFGITR.ERET.
>
> Ah, so that's the register, thanks!
>
> I stil am not sure that having FEAT_SPE, an Armv8.3 extension, depend
> on
> FEAT_FGT, an Armv8.6 extension, is the best idea. Do you know of any
> machines
> that have FEAT_SPE and FEAT_FGT?

None. Both are pretty niche, and the combination is nowhere
to be seen at the moment.

> On the plus side, KVM could enable the trap only in the case above, and
> disable
> it after the ERET is trapped, so it should be relatively cheap to use.

This feels pretty horrible. Nothing says *when* will EL1
alter the PTs. It could take tons of EL1->EL1 exceptions
before returning to EL0. And the change could happen after
an EL1->EL0->EL1 transition. At which point do you stop?

If you want to rely on ERET for that, you need to trap
ERET all the time, because all ERETs to EL0 will be
suspect. And doing that to handle such a corner case feels
pretty horrible.

M.
--
Jazz is not dead. It just smells funny...