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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <ZN6w2SxyZMKKxtb/@google.com>
Date:   Thu, 17 Aug 2023 16:44:25 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kyle Meyer <kyle.meyer@....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>,
        Russ Anderson <russ.anderson@....com>,
        Dimitri Sivanich <dimitri.sivanich@....com>,
        Steve Wahl <steve.wahl@....com>
Subject: Re: [PATCH] KVM: x86: Increase KVM_MAX_VCPUS to 4096

On Thu, Aug 17, 2023, Kyle Meyer wrote:
> > > 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.

Yeah, that's probably for the best.