Re: [PATCH 2/2] KVM: selftests: arm64: Explicitly set the page attrs to Inner-Shareable

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

On Fri, Apr 04, 2025 at 10:06:59PM +0000, Raghavendra Rao Ananta wrote:
> Atomic instructions such as 'ldset' over (global) variables in the guest
> is observed to cause an EL1 data abort with FSC 0x35 (IMPLEMENTATION
> DEFINED fault (Unsupported Exclusive or Atomic access)). The observation
> was particularly apparent on Neoverse-N3.
> 
> According to DDI0487L.a C3.2.6 (Single-copy atomic 64-byte load/store),
> it is implementation defined that a data abort with the mentioned FSC
> is reported for the first stage of translation that provides an
> inappropriate memory type. It's likely that the same rule also applies
> to memory attribute mismatch. When the guest loads the memory location of
> the variable that was already cached during the host userspace's copying
> of the ELF into the memory, the core is likely running into a mismatch
> of memory attrs that's checked in stage-1 itself, and thus causing the
> abort in EL1.

Sorry, my index of the ARM ARM was trashed when we were discussing this
before.

DDI0487L.a B2.2.6 describes the exact situation you encountered, where
atomics are only guaranteed to work on Inner/Outer Shareable MT_NORMAL
memory.

What's a bit more explicit for other memory attribute aborts (like the
one you've cited) is whether or not the implementation can generate the
abort solely on the stage-1 attributes vs. the combined stage-1/stage-2
attributes at the end of translation.

Either way, let's correct the citation to point at the right section.

Thanks,
Oliver