linux-kernel - Re: [PATCH v3 4/4] x86/kvm: add boot parameter for setting max number of vcpus per guest

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Message-ID: <YZZ1W+YxagvG1EMM@google.com>
Date:   Thu, 18 Nov 2021 15:46:35 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 4/4] x86/kvm: add boot parameter for setting max
 number of vcpus per guest

On Thu, Nov 18, 2021, Juergen Gross wrote:
> On 18.11.21 16:05, Sean Christopherson wrote:
> > Which is a good segue into pointing out that if a module param is added, it needs
> > to be sanity checked against a KVM-defined max.  The admin may be trusted to some
> > extent, but there is zero reason to let userspace set max_vcspus to 4 billion.
> > At that point, it really is just a param vs. capability question.
> 
> I agree. Capping it at e.g. 65536 would probably be a good idea.

Any reason to choose 65536 in particular?  Why not cap it at the upper limit of
NR_CPUS_RANGE_END / MAXSMP, which is currently 8192?