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

[Borislav Petkov <bp@suse.de>]

On Thu, Dec 10, 2015 at 11:01:35AM +0000, Will Deacon wrote:
> [adding Boris, as he might know how this works]

Gee, thanks Will, now you're making me look at this too :-)

> It's not about flushing one page, flush_tlb_kernel_range (which is called
> by unmap_kernel_range) already takes care of that. The problem is that
> the unmap is called from irq/nmi context, so the IPIs required for
> broadcasting the TLB maintenance on x86 cannot be safely executed.

Hmm, if you're talking about ghes_iounmap_nmi() and ghes_iounmap_irq()
which are the two callers of unmap_kernel_range_noflush(), that last one
is calling vunmap_page_range() which is fiddling with the page table.
And I don't see TLB flushing IPIs there.

If you mean arch_apei_flush_tlb_one(), that's INVLPG on x86 so also no
IPI.

What am I missing?

> Ideally, I think the ghes code would just use unmap_kernel_range unless
> the architecture specifically says that doesn't work in irq context. In
> that case, we don't need to implement the arch_apei_flush_tlb_one callback
> on arm64.

Well, what bothers me with using
unmap_kernel_range()/vunmap_page_range() is that if a GHES IRQ/NMI
happens while something is executing those, the NMI will interrupt
whatever's happening and it will possibly corrupt the pagetable, IMHO.

Michal, Vlasta, can you please take a look?

More specifically, those ghes_iounmap_nmi/ghes_iounmap_irq calls to
unmap_kernel_range_noflush() happening in NMI/IRQ context.

> One thing I don't fully grok about the code: since the page is mapped
> using ioremap_page_range, doesn't that allow other CPUs to speculatively
> fill their TLB with entries corresponding to the page mapped by the IRQ
> handler on another core? If the core with the speculative entries then
> takes an APEI exception, what guarantees do we have that it's looking at
> the right page? I think, for x86, we need a local invalidation on map,
> too.

You're looking at ghes_copy_tofrom_phys(), right? That's grabbing
spinlocks in IRQ/NMI context and doing the iounmap a bit later, below
on the same core. I mean, I don't see us landing on another core in
between, we're non-preemptible...

Or do you mean something else?

-- 
Regards/Gruss,
    Boris.

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