Re: [Xen-devel] [PATCH v6 05/12] microcode: only save compatible ucode patches

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.03.19 at 18:03, <roger.pau@xxxxxxxxxx> wrote:
> On Mon, Mar 11, 2019 at 03:57:29PM +0800, Chao Gao wrote:
>> Intel CPU only allows mixing in stepping or 'pf'. If an ucode patch is
>> for other CPU families or models, it won't be compatible to all CPUs on
>> current system and even hot-plugged CPUs.  Don't save such patch to
>> reduce the size of ucode cache.
> 
> Oh, so we are indeed aiming to support systems that can require CPUs
> to use different microcode blobs in order to update?
> 
> If that's the case, and we are sure this will work, we do indeed need
> a list of microcodes, but you will have to somehow flag applied
> microcodes in order to prevent them from being replaced with new
> microcode versions until Xen knows that such new versions apply
> correctly.
> 
> Maybe I'm being too simplistic here, but does anyone really have a
> system with CPUs with different stepping?

I think the primary scenario is to see a newer stepping CPU added
into a slightly older system (hotplug or not). Whether people
actually do this I don't know, but I suspect there's a reason hw
vendors state what combinations are supported.

> Supporting such scenario adds quite a lot of complexity and even if
> theoretically possible I don't think it makes much sense to support
> unless there are such systems out there.

What complexity are you thinking about? Of course the expectation
is for hotplugged CPUs to have all features already present CPUs
have. And of the ones we find at boot we settle on a common
minimum set of features (or at least we try to), but in mixed-
stepping systems the common situation would anyway be that
they're all equal in terms of software visible features.

Jan



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