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

[Ryan Roberts <ryan.roberts@arm.com>]

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