Re: [Xen-devel] [PATCH] x86/boot: Make alternative patching NMI-safe

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.02.18 at 19:04, <andrew.cooper3@xxxxxxxxxx> wrote:
> But as you correctly pointed out, it was a very long way from being
> complete.  We currently have no idea whether we are in NMI context, so
> arranging not to not execute an iret is hard.

As long as we don't mean to patch extremely early or extremely
late parts of the handler, why couldn't the NMI handler set a flag
(or increment some sort of counter; perhaps in struct cpu_info),
which the exit path inspects (and clears under the appropriate
conditions)?

Jan


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