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Xen 4.12 isn't > >>>>>> relevant. > >>>>>> > >>>>>> Expand and correct the commentary. > >>>>>> > >>>>>> Fixes: 111c8c33a8a1 ("x86/cpuid: do not expand max leaves on restore") > >>>>> But doesn't this commit amend 685e922d6f30 ("tools/libxc: Rework > >>>>> xc_cpuid_apply_policy() to use {get,set}_cpu_policy()"), which is > >>>>> where DEF_MAX_* disappeared? > >>>> No. All that happened in that change was that we switched to using > >>>> > >>>> cpuid.h:89:#define CPUID_GUEST_NR_EXTD_AMD > >>>> > >>>> instead, which remained the same size until Xen 4.15 when e9b4fe26364 > >>>> bumped it. > >>> Oh, right. I did try to look for a replacement, but managed to miss > >>> this. But then, as much as 4.12 isn't relevant, isn't it the case > >>> that the fact that CPUID data was added to the stream in 4.14 isn't > >>> relevant here either, and it's instead the bumping in 4.15 which is? > >> The fact that the bump happened is relevant, by virtue of the fact there > >> logic added to cope.  The fact it was in 4.15 is not relevant - this > >> isn't a list of every ABI-relevant change. > >> > >> CPUID data being added to the stream is critically important, because > >> that's the point after which we never enter this compatibility path. > > If the bump happened before CPUID data was added to the stream, logic to > > cope with migrating-in guests would have been required too, wouldn't it. > > Yes, it would have been. > > It wasn't an accident that none of the max leaves changed while doing > the Xen CPUID work. > > We're unfortunately a long way behind on Intel CPUID leaves, but all(?) > of the new leaves need more complicated migration safely logic than the > toolstack currently knows how to do. > > > But anyway, just to be done with this: > > Acked-by: Jan Beulich > > Thanks. Will rebase my CPUID series on top of this, but I will wait for further comments before sending a new version. Roger.