Re: [PATCH 2/2] KVM: arm64: Destroy the stage-2 page-table periodically

[Oliver Upton]

On Thu, Aug 07, 2025 at 11:58:01AM -0700, Raghavendra Rao Ananta wrote:
> Hi Oliver,
> 
> >
> > Protected mode is affected by the same problem, potentially even worse
> > due to the overheads of calling into EL2. Both protected and
> > non-protected flows should use stage2_destroy_range().
> >
> I experimented with this (see diff below), and it looks like it takes
> significantly longer to finish the destruction even for a very small
> VM. For instance, it takes ~140 seconds on an Ampere Altra machine.
> This is probably because we run cond_resched() for every breakup in
> the entire sweep of the possible address range, 0 to  ~(0ULL), even
> though there are no actual mappings there, and we context switch out
> more often.

This seems more like an issue with the upper bound on a pKVM walk rather
than a problem with the suggestion. The information in pgt->ia_bits is
actually derived from the VTCR value of the owning MMU.

Even though we never use the VTCR value in hardware, pKVM MMUs have a
valid VTCR value that encodes the size of the IPA space and we use that
in the common stage-2 abort path.

I'm attaching some fixups that I have on top of your series that'd allow
the resched logic to remain common, like it is in other MMU flows.