Re: [PATCH 12/18] arm64: Implement try_populate_vmemmap_pmd using AF trick

On Wed, Jul 08, 2026 at 03:11:22AM +0000, James Houghton wrote:
> This routine is to be used for updating in-use, leaf-level PMDs in the
> vmemmap without introducing a window where vmemmap accesses might fault.
> 
> Implementing this on arm64 requires some care: use the same access flag
> trick that is used for vmemmap PTE updates. HAFT is not needed and the
> TLB flushing routine remains identical, as we are not overwriting a
> non-leaf PMD.
> 
> For systems that support BBML2_NOABORT, there is no need to use the AF
> trick.

This part needs even more text in the Arm ARM. With dropping of the
BBML2_NOABORT requirement in patch 13, we are left with the D8.17.2
rules in the Arm ARM (M.c) requiring BBM. I'm not sure we can infer much
from the AF=0 caching rules as here we talk about the walk caches. For
example, a walk cache memorises the VA to block descriptor translation
and that's irrespective of the leaf entry AF bit. If AF=0 and you skip
the TLBI, nothing flushes the walk cache which may confuse the CPU when
it suddenly sees a table entry where it expected a block one.

If it's not critical to the series, I suggest we leave this patch and
the next one out for now.

-- 
Catalin