linux-kernel - Re: [PATCH] KVM: x86: Increase KVM_MAX

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Date:   Thu, 17 Aug 2023 20:23:16 +0000
From:   "Meyer, Kyle" <kyle.meyer@....com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hasen@...ux.intel.com" <dave.hasen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "Anderson, Russ" <russ.anderson@....com>,
        "Sivanich, Dimitri" <dimitri.sivanich@....com>,
        "Wahl, Steve" <steve.wahl@....com>
Subject: Re: [PATCH] KVM: x86: Increase KVM_MAX_VCPUS to 4096

> > 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.