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Re: [Xen-devel] [PATCH] x86/AMD: Add support for AMD's OSVW feature in guests

On 01/19/12 03:10, Jan Beulich wrote:

Btw., one more comment on the change to init_amd(): You will likely
need to distinguish BP and AP cases here - on the BP you want to
plainly write the values read from the MSRs to the global variables,
but on APs (with possibly different settings) you need to work
towards a setting of the global variables that apply to all of the
CPUs. This is not just for the (theoretical only?) hotplug case, but
particularly the one of a soft-offlined CPU that had its microcode
updated already in a way affecting the OSVW bits and is
subsequently being brought back online.

Not sure I follow you. There is no difference in how BIOS/microcode sets OSVW bits on AP or BP, that's based on silicon version (i.e. stepping) and not where that silicon is plugged in.

If a core is offlined that will presumably happen after it has already gone through init_amd() and therefore global versions of the registers already accounted for that processor's values. (The downside is that if the offlined processors has more bugs than the rest then everyone is considered as bad even though the bad processor is out of the action. But I think the positives of updating global variables after a cpu is offlined are not worth the complexity.)

Which reminds you and me that the patch is missing integration with
the microcode update loader (as that one may alter the OSVW bits

That's a good point --- I should re-initialize the global variables after a patch is loaded.


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