Re: [Xen-devel] [PATCH] x86/nmi: Fix shootdown of pcpus running in VMX non-root mode

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.02.15 at 12:52, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 09/02/15 11:43, Tim Deegan wrote:
>> Hi,
>>
>> At 11:25 +0000 on 09 Feb (1423477508), Andrew Cooper wrote:
>>> In the case of a crash, nmi_shootdown_cpus() patches nmi_crash() into the 
> IDT
>>> of each processor, such that the next NMI it receives will force it into the
>>> crash path.
>>>
>>> c/s 7dd3b06ff "vmx: fix handling of NMI VMEXIT" fixed one issue but
>>> inadvertently introduced another.  The original use of self_nmi() would 
> follow
>>> vector #2, but a direct call to do_nmi() does not.
>>>
>>> Introduce a function pointer which should be used in preference to direct
>>> do_nmi() calls, which is updated on the crash path to point at 
> do_nmi_crash()
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>>> CC: Tim Deegan <tim@xxxxxxx>
>>>
>>> ---
>>>
>>> This patch very certainly functions correctly (it is in active use now in a
>>> customer escalation), but I was wondering how paranoid we should be about
>>> interleaved reads/writes and whether an atomic write would be better?
>>> Performance is not a issue at all but in a crash senario we don't want to be
>>> taking any chances with correctness.
>> Yes, atomic updates sound like a good idea.  Would it make sense to
>> add a _get_gate() or similar so the vmx path can read the actual IDT
>> rather than adding a _third_ place where we set what to do on NMI?
> 
> A _get_gate() would return nmi() or nmi_crash() rather than do_nmi() or
> do_nmi_crash().  The latter pair is needed as we are already executing
> in C context rather than coming straight in from an interrupt.

So wouldn't it be possible to get rid of nmi_crash() and have
nmi() call *nmi_handler instead of don_nmi (and nmi_handler
would really just become an alias of exception_table[2]?

Jan


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