Re: [Xen-devel] [PATCH v1 for 4.13] x86/microcode: refuse to load the same revision ucode

[Chao Gao <chao.gao@xxxxxxxxx>]

On Fri, Nov 22, 2019 at 12:19:41PM +0100, Jan Beulich wrote:
>On 22.11.2019 11:52, Sergey Dyasli wrote:
>> Currently if a user tries to live-load the same ucode revision that CPU
>> already has, he will get a single message in Xen log like:
>> 
>>     (XEN) 128 cores are to update their microcode
>> 
>> No actual ucode loading will happen and this situation can be quite
>> confusing. Fix this by starting ucode update only when a newer ucode
>> revision has been provided. This is based on an assumption that all CPUs
>> in the system have the same ucode revision. If that's not the case,
>> the system must be considered unstable.
>
>Unstable or not, I did specifically convince Chao to handle such
>systems, bringing them into better shape. I can only repeat that
>I actually have a system where on each socket firmware loads ucode
>only on the first core. I don't see why boot time loading and late
>loading should differ in behavior for such a system.

Yes. 

I tried to load an older ucode but also got the same message. So I think
an optimization can be done:
we can assume that if there is a microcode_cache, all CPUs
should have equal or newer revision than the microcode_cache. If
the patch to be loaded has equal or older revision, we can refuse to
load to avoid the heavy stop_machine().

Thanks
Chao

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