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

Suzuki K Poulose
2026-04-09T09:38:07Z

On 07/04/2026 18:21, Catalin Marinas wrote:
> On Tue, Apr 07, 2026 at 10:57:35AM +0100, Suzuki K Poulose wrote:
>> 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().
>>>
>>> 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).
>>
>> That is correct. map_mem() comes from paginig_init(), which gets called
>> before arm64_rsi_init(). Realm check was delayed until psci_xx_init().
>> We had a version which parsed the DT for PSCI conduit early enough
>> to be able to make the SMC calls to detect the Realm. But there
>> were concerns around it.
>
> Ah, yes, I remember.
>
> Does it mean that commit 42be24a4178f ("arm64: Enable memory encrypt for
> Realms") was broken without rodata=full w.r.t. the linear map? Commit

Apparently, it looks like we missed this when we demoted the RSI
detection later.

> a166563e7ec3 ("arm64: mm: support large block mapping when rodata=full")
> introduced force_pte_mapping() but it just copied the logic in the
> existing can_set_direct_map(). Looking at the linear_map_requires_bbml2
> assignment, we get (!is_realm_world() && is_realm_world()) and it
> cancels out, no effect on it but we don't get pte mappings either (even
> if we don't have BBML2).

Yep, that's right.
>
> I think we need at least some safety checks:
>
> 1. BBML2_NOABORT support on the boot CPU - continue with the existing
> logic (as per Ryan's series)
>
> 2. !system_supports_bbml2_noabort() - split in
> linear_map_maybe_split_to_ptes(). This does not currently happen
> because linear_map_requires_bbml2 may be false in the absence of
> rodata=full. Not sure how to fix this without some variable telling
> us how the linear map was mapped. The requires_bbml2 flag doesn't
>
> 3. Panic in arm64_rsi_init() if !BBML2_NOABORT on the boot CPU _and_ we
> have block mappings already. People can avoid it with rodata=full

It looks like this will be a common case :-(

>
> 4. If (3) is a common case, a better alternative is to rewrite the
> linear map sometime after arm64_rsi_init() but before we call
> split_kernel_leaf_mapping().

We will explore this route.

The other option is to move the RSI detection (and the PSCI probe)
earlier to be able to make better decisions early on. I will play with
that a bit too.

Suzuki

>