Messages in this thread | | | Date | Mon, 16 Jun 2014 15:12:42 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] arm64: Add flush_cache_vmap call in __early_set_fixmap |
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On Mon, Jun 09, 2014 at 02:24:29PM +0100, Leif Lindholm wrote: > On Mon, Jun 09, 2014 at 12:03:56PM +0100, Catalin Marinas wrote: > > So I'm proposing an alternative patch (which needs some benchmarking as > > well to see if anything is affected, maybe application startup time).
I don't like the proposed patch at all -- keeping the dsb out of set_pte is worthwhile if we can manage it. That said, it would be interesting to know how often we get a subsequent page fault after mapping invalid -> valid because of the missing dsb. It could be that the cost of the benign fault is hitting us more than we think.
> I'm happy for any fix which can be included in 3.16. > > But is the dsb(ishst) sufficient? We need to also prevent reads from > overtaking the set_pte(). i.e.: > > ptr = early_ioremap(phys_addr, size); > if (ptr && strcmp(ptr, "magic") == 0) > ... > > Does it not require a dsb(ish)?
I don't think so. Crudely, the sequence above would look like:
STR x0, [PTEP] DSB ISHST LDR x0, [MAGIC]
The DSB can't complete until the STR is globally observed within the inner-shareable domain, so the LDR cannot execute until the page table update is visible to the walker.
If it was a DMB, we'd have a problem. Interestingly, the asm-generic page table allocators (e.g. __pmd_alloc) *do* use dmb for ordering observability of page-table updates via smp_wmb. I'm struggling to decide whether that's broken or not.
Will
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