Re: [PATCH 02/17] arm64/mm: Convert READ_ONCE() as pmdp_get() while accessing PMD

[David Hildenbrand (Arm)]

On 7/29/26 14:24, Anshuman Khandual wrote:
> Convert all READ_ONCE() based PMD accesses as pmdp_get() instead which will
> support both D64 and D128 translation regime going forward. That is because
> READ_ONCE() would need 128 bit single copy atomic guarantees, while reading
> 128 bit page table entries which is currently not supported on arm64. Build
> fails for READ_ONCE() while accessing beyond 64 bits.
> 
> Load Pair/Store Pair (ldp/stp) are only single copy atomic if FEAT_LSE128
> is supported (which is required when FEAT_D128 is supported). Currently 128
> bit pgtables is a compile time decision - so we could have chosen to extend
> READ_ONCE()/WRITE_ONCE() to allow 128 bit for this configuration. But then
> it's a general purpose API and we were concerned that other users might
> eventually creep in that expect 128 and then fail to compile in the other
> configs.
> 
> But worse, we are considering eventually making D128 a boot time option, at
> which point we'd have to make READ_ONCE() always allow 128 bit at compile
> time but then it might silently tear at runtime.
> 
> So our preference is to standardize on these existing helpers, which we can
> override in arm64 to give the 128 bit single copy guarantee when required.
> 

Best to mention here that no functional change is expect because the common-cde
pmdp_get() etc helpers do exactly that.

Reviewed-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>

-- 
Cheers,

David