Re: [PATCH] KVM: x86: Increase KVM_MAX_VCPUS to 4096

["Meyer, Kyle" <kyle.meyer@hpe.com>]

> > 4096 is the current maximum value because of the Hyper-V TLFS. See
> > BUILD_BUG_ON in arch/x86/kvm/hyperv.c, commit 79661c3, and Vitaly's
> > comment on https://lore.kernel.org/all/87r136shcc.fsf@redhat.com.
>
> Mostly out of curiosity, do you care about Hyper-V support?   If not, at some
> point it'd probably be worth exploring a CONFIG_KVM_HYPERV option to allow
> disabling KVM's Hyper-V support at compile time so that we're not bound by the
> restrictions of the TLFS.

Yes, I care about Hyper-V support. I would like this limitation to be addressed
in the future.

> Rather than tightly couple this to MAXSMP, what if we add a Kconfig?  I know of
> at least one scenario, SVM's AVIC/x2AVIC, where it would be desirable to configure
> KVM to a much smaller maximum.  The biggest downside I can think of is that KVM
> selftests would need to be updated (they assume the max is >=512), and some of the
> tests might be completely invalid if KVM_MAX_VCPUS is too low (<256?).

That sounds good to me. I would prefer to set the range from 1024 to 4096 in
this patch.