Re: [PATCH v2 1/3] arm64: mm: Fix rodata=full block mapping support for realm guests

Catalin Marinas
2026-04-02T20:44:04Z

On Mon, Mar 30, 2026 at 05:17:02PM +0100, Ryan Roberts wrote:
> int split_kernel_leaf_mapping(unsigned long start, unsigned long end)
> {
> int ret;
>
> - /*
> - * !BBML2_NOABORT systems should not be trying to change permissions on
> - * anything that is not pte-mapped in the first place. Just return early
> - * and let the permission change code raise a warning if not already
> - * pte-mapped.
> - */
> - if (!system_supports_bbml2_noabort())
> - return 0;
> -
> /*
> * If the region is within a pte-mapped area, there is no need to try to
> * split. Additionally, CONFIG_DEBUG_PAGEALLOC and CONFIG_KFENCE may
> * change permissions from atomic context so for those cases (which are
> * always pte-mapped), we must not go any further because taking the
> - * mutex below may sleep.
> + * mutex below may sleep. Do not call force_pte_mapping() here because
> + * it could return a confusing result if called from a secondary cpu
> + * prior to finalizing caps. Instead, linear_map_requires_bbml2 gives us
> + * what we need.
> */
> - if (force_pte_mapping() || is_kfence_address((void *)start))
> + if (!linear_map_requires_bbml2 || is_kfence_address((void *)start))
> return 0;
>
> + if (!system_supports_bbml2_noabort()) {
> + /*
> + * !BBML2_NOABORT systems should not be trying to change
> + * permissions on anything that is not pte-mapped in the first
> + * place. Just return early and let the permission change code
> + * raise a warning if not already pte-mapped.
> + */
> + if (system_capabilities_finalized())
> + return 0;
> +
> + /*
> + * Boot-time: split_kernel_leaf_mapping_locked() allocates from
> + * page allocator. Can't split until it's available.
> + */
> + if (WARN_ON(!page_alloc_available))
> + return -EBUSY;
> +
> + /*
> + * Boot-time: Started secondary cpus but don't know if they
> + * support BBML2_NOABORT yet. Can't allow splitting in this
> + * window in case they don't.
> + */
> + if (WARN_ON(num_online_cpus() > 1))
> + return -EBUSY;
> + }

I think sashiko is over cautions here
(https://sashiko.dev/#/patchset/20260330161705.3349825-1-ryan.roberts@arm.com)
but it has a somewhat valid point from the perspective of
num_online_cpus() semantics. We have have num_online_cpus() == 1 while
having a secondary CPU just booted and with its MMU enabled. I don't
think we can have any asynchronous tasks running at that point to
trigger a spit though. Even async_init() is called after smp_init().

An option may be to attempt cpus_read_trylock() as this lock is taken by
_cpu_up(). If it fails, return -EBUSY, otherwise check num_online_cpus()
and unlock (and return -EBUSY if secondaries already started).

Another thing I couldn't get my head around - IIUC is_realm_world()
won't return true for map_mem() yet (if in a realm). Can we have realms
on hardware that does not support BBML2_NOABORT? We may not have
configuration with rodata_full set (it should be complementary to realm
support).

I'll add the patches to for-next/core to give them a bit of time in
-next but let's see next week if we ignore this (with an updated
comment) or we try to avoid the issue altogether.

--
Catalin