Re: [Xen-devel] [PATCH v9 15/15] microcode: block #NMI handling when loading an ucode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.09.2019 07:52, Chao Gao wrote:
> On Fri, Aug 30, 2019 at 02:35:06PM +0800, Chao Gao wrote:
>>> Doing the actual ucode update inside an NMI handler seems rather risky
>>> to me. Even if Ashok confirmed it would not be an issue on past and
>>> current Intel CPUs - what about future ones, or ones from other vendors?
>>
> 
> Intel SDM doesn't say that loading ucode isn't allowed inside an NMI
> handler. So it is allowed implicitly.

Well, if the SDM was complete and correct everywhere else, I'd agree
to such an interpretation / implication.

> If future CPUs cannot load ucode
> in NMI handler, SDM should document it and at that time, we can move
> ucode loading out of NMI handler for new CPUS. As to AMD, if someone
> objects to this approach, let's use this approach for Intel only.

Getting a definitive statement may turn out difficult. But I guess if
you support both approaches anyway, having a command line option to
override the default behavior wouldn't be much more than a 1 line
addition?

Jan

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