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 Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) -- -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html