Re: [PATCH v2 07/14] arm64/mm: Avoid barriers for invalid or userspace mappings

[Kevin Brodsky <kevin.brodsky@arm.com>]

On 17/02/2025 15:07, Ryan Roberts wrote:
> __set_pte_complete(), set_pmd(), set_pud(), set_p4d() and set_pgd() are

Nit: it would be more accurate to say __set_pte() instead of
__set_pte_complete(), as it is the former that actually writes the PTE
(and then issues barriers).

> used to write entries into pgtables. And they issue barriers (currently
> dsb and isb) to ensure that the written values are observed by the table
> walker prior to any program-order-future memory access to the mapped
> location.
>
> Over the years some of these functions have received optimizations: In
> particular, commit 7f0b1bf04511 ("arm64: Fix barriers used for page
> table modifications") made it so that the barriers were only emitted for
> valid-kernel mappings for set_pte() (now __set_pte_complete()). And
> commit 0795edaf3f1f ("arm64: pgtable: Implement p[mu]d_valid() and check
> in set_p[mu]d()") made it so that set_pmd()/set_pud() only emitted the
> barriers for valid mappings. set_p4d()/set_pgd() continue to emit the
> barriers unconditionally.
>
> This is all very confusing to the casual observer; surely the rules
> should be invariant to the level? Let's change this so that every level
> consistently emits the barriers only when setting valid, non-user
> entries (both table and leaf).
>
> It seems obvious that if it is ok to elide barriers all but valid kernel
> mappings at pte level, it must also be ok to do this for leaf entries at
> other levels: If setting an entry to invalid, a tlb maintenance
> operation must surely follow to synchronise the TLB and this contains
> the required barriers. If setting a valid user mapping, the previous
> mapping must have been invalid and there must have been a TLB
> maintenance operation (complete with barriers) to honour
> break-before-make. So the worst that can happen is we take an extra
> fault (which will imply the DSB + ISB) and conclude that there is
> nothing to do. These are the arguments for doing this optimization at
> pte level and they also apply to leaf mappings at other levels.
>
> For table entries, the same arguments hold: If unsetting a table entry,
> a TLB is required and this will emit the required barriers. If setting a

s/TLB/TLB maintenance/

> table entry, the previous value must have been invalid and the table
> walker must already be able to observe that. Additionally the contents
> of the pgtable being pointed to in the newly set entry must be visible
> before the entry is written and this is enforced via smp_wmb() (dmb) in
> the pgtable allocation functions and in __split_huge_pmd_locked(). But
> this last part could never have been enforced by the barriers in
> set_pXd() because they occur after updating the entry. So ultimately,
> the wost that can happen by eliding these barriers for user table

s/wost/worst/

- Kevin

> entries is an extra fault.
>
> [...]