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Re: [Xen-devel] [PATCH for-4.12] x86/svm: Fix handling of ICEBP intercepts

>>> On 01.02.19 at 18:09, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 01/02/2019 16:55, Jan Beulich wrote:
>>>>> On 01.02.19 at 17:25, <andrew.cooper3@xxxxxxxxxx> wrote:
>>> If it were just getting insn_len incorrectly as 0, then the guest would
>>> livelock as we wouldn't inject the #DB with trap semantics it requires,
>> I'm confused again: Why trap semantics? The ICEBP has fault
>> semantics as you confirmed above.
> The ICEBP intercept has fault semantics.  An ICEBP instruction executing
> in the guest has trap semantics.

Oh, okay - I was mis-remembering this aspect.

>>> but as the #GP is already raised, this will combine to #DF.
>> How that? #DB is a benign exception, so according to the table on the
>> #DF page in the SDM, with #GP it shouldn't combine to #DF.
> #GP is raised first.  It is contributory.
> A subsequent #DB getting raised causes #GP to turn into #DF.

That's based on what? The table on the #DF page clearly says
otherwise, at least according to my reading.

But in the end there shouldn't be any attempt to inject #DB anyway
when #GP is already pending, irrespective of the fact that this #GP
is non-architectural.


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