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SubjectRe: [PATCH V15 06/11] acpi: apei: handle SEA notification type for ARMv8
Hi Tyler,

On 19/04/17 00:05, Tyler Baicar wrote:
> ARM APEI extension proposal added SEA (Synchronous External Abort)
> notification type for ARMv8.
> Add a new GHES error source handling function for SEA. If an error
> source's notification type is SEA, then this function can be registered
> into the SEA exception handler. That way GHES will parse and report
> SEA exceptions when they occur.
> An SEA can interrupt code that had interrupts masked and is treated as
> an NMI. To aid this the page of address space for mapping APEI buffers
> while in_nmi() is always reserved, and ghes_ioremap_pfn_nmi() is
> changed to use the helper methods to find the prot_t to map with in
> the same way as ghes_ioremap_pfn_irq().

> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index b74d8b7..10013ff 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -518,6 +520,17 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> pr_err("Synchronous External Abort: %s (0x%08x) at 0x%016lx\n",
> inf->name, esr, addr);
> + /*
> + * Synchronous aborts may interrupt code which had interrupts masked.
> + * Before calling out into the wider kernel tell the interested
> + * subsystems.
> + */
> + nmi_enter();
> + ghes_notify_sea();
> + nmi_exit();
> + }
> +
> info.si_signo = SIGBUS;
> info.si_errno = 0;
> info.si_code = 0;

I was tidying up the masking/unmasking in entry.S, something I wasn't aware of
that leads to a bug:
entry.S will unmask interrupts for instruction/data aborts that came from a
context with interrupts enabled. This makes sense for get_user() and friends...
For do_sea() we pull nmi_enter() as this can interrupt interrupts-masked code,
such as APEI, but if we end up in here with interrupts unmasked we can take an
IRQ from this 'NMI' context, which will inherit the in_nmi() and could lead to
the deadlock we were originally trying to avoid.

Teaching entry.S to spot external aborts is messy. I think the two choices are
to either mask interrupts when calling nmi_enter() (as these things should be
mutually exclusive), or to conditionally call nmi_enter() based on
interrupts_enabled(regs). I prefer the second one as it matches the notify_sea()
while interruptible that happens when KVM takes one of these.



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