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

Ryan Roberts
2026-04-07T08:33:56Z

On 02/04/2026 21:43, Catalin Marinas wrote:
> 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().

Yes I saw the Sashiko report, but we had previously had a (private) discussion
where I thought we had already concluded that this approach is safe in practice
due to the way that the boot cpu brings the secondaries online.

>
> 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).

That sounds neat; I could dig deeper and have a go at something like this if you
want?

>
> 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).

My understanding is that this is a pre-existing (and known) bug. It's not
related to the "map linear map by large leaves and split dynamically" feature so
wasn't attempting to fix it.

I had heard that in practice all FEAT_RME systems should support FEAT_BBML3
which would solve the problem. Not sure how true that is though.

>
> 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.
>

Thanks,
Ryan