Re: [PATCH v4] acpi, apei, arm64: APEI initial support for aarch64.

[Borislav Petkov]

On Mon, Dec 14, 2015 at 11:46:59AM +0000, Will Deacon wrote:
> We're in violent agreement. I'm just saying that's *why*
> arch_apei_flush_tlb_one exists, as opposed to calling unmap_kernel_range
> in the driver (which will attempt IPIs). On arm64, unmap_kernel_range
> will actually work correctly, since we don't need IPIs to broadcast TLB
> maintenance.
> 
> The (incorrect) premise earlier in the thread was that
> arch_apei_flush_tlb_one exists because there's no portable API for
> flushing a single page, but that's simply not true.

Right.

> Yikes, I'd not even thought about that. Perhaps its all serialised
> somehow, but I have no idea.

Yeah, didn't see any serialization there...

> Right, imagine the following sequence of events:
> 
>  1. CPU x takes a GHES IRQ
>  2. CPU x then maps the buffer a page at a time in ghes_copy_tofrom_phys.
>     After each unmap, it performs a local TLBI. Let's say that it has
>     the final page of the buffer mapped when...
>  3. ... CPU y is meanwhile happily executing some other kernel code.
>  4. CPU y's page table walker speculatively fills the TLB with a translation
>     for the last buffer page that CPU x has mapped (because its just been
>     mapped with ioremap_page_range and is in the kernel page table).
>  5. CPU x unmaps the last page, performs a *local* TLBI, handles the
>     event and returns from the exception
>  6. CPU y takes a GHES IRQ
>  7. CPU y then maps the first buffer page at the same virtual address
>     that CPU x used to map the last buffer page
>  8. CPU y accesses the page, hits the stale TLB entry and gets junk
> 
> which I think means you need to perform local TLB invalidation on map
> as well as unmap.
> 
> Is there some reason this can't happen on x86? It sounds plausible on
> arm64 if we were to use local invalidation.

Ha, thanks for the detailed example, I see it now!

And I too don't see a reason why that can't happen. And the GHES
IRQ is a GSI, which has "global" in the name but I don't think that
means it interrupts the whole system like an NMI does. Especially
if it is registered/handled like a normal irq: acpi_gsi_to_irq() ..
request_irq()...

Adding Tony.

If anything, we probably should be doing something with irq_work at the
end of ghes_copy_tofrom_phys() so that the invalidation of any possible
speculative mappings happens before we return from the GHES IRQ.

Hmm, currently I'm not even clear whether this'll work: we would
theoretically need to send IPIs from IRQ context, at the end of the GHES
IRQ...

Thanks.

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix ImendÃrffer, Jane Smithard, Graham Norton, HRB 21284 (AG NÃrnberg)
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