Messages in this thread | | | Date | Wed, 10 Apr 2024 12:06:04 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2 2/4] arm64: mm: Batch dsb and isb when populating pgtables |
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On Wed, Apr 10, 2024 at 11:25:10AM +0100, Ryan Roberts wrote: > On 10/04/2024 11:06, Mark Rutland wrote: > > On Thu, Apr 04, 2024 at 03:33:06PM +0100, Ryan Roberts wrote: [> >> @@ -234,6 +238,13 @@ static void alloc_init_cont_pte(pmd_t *pmdp, unsigned long addr, > >> } while (addr = next, addr != end); > >> > >> pte_clear_fixmap(); > >> + > >> + /* > >> + * Ensure all previous pgtable writes are visible to the table walker. > >> + * See init_pte(). > >> + */ > >> + dsb(ishst); > >> + isb(); > > > > Hmm... currently the call to pte_clear_fixmap() alone should be sufficient, > > since that needs to update the PTE for the fixmap slot, then do maintenance for > > that. > > Yes, true... > > > > > So we could avoid the addition of the dsb+isb here, and have a comment: > > > > /* > > * Note: barriers and maintenance necessary to clear the fixmap slot > > * ensure that all previous pgtable writes are visible to the table > > * walker. > > */ > > pte_clear_fixmap(); > > > > ... which'd be fine as long as we keep this fixmap clearing rather than trying > > to do that lazily as in patch 4. > > But it isn't patch 4 that breaks it, it's patch 3. Once we have abstracted > pte_clear_fixmap() into the ops->unmap() call, for the "late" ops, unmap is a > noop.
Ah, yep; I hadn't spotted that yet.
> I guess the best solution there would be to require that unmap() always > issues these barriers. > > I'll do as you suggest for this patch. If we want to keep patch 3, then I'll add > the barriers for all unmap() impls.
Thanks. It's going to take me a bit longer to chew through patches 3 and 4, but I will try to get through those soon.
For now a slightly simpler option would be to have patch 3 introduce the DSB+ISB as above rather than in each of the unmap() impls.
Mark.
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