linux-kernel - Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits

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Message-ID: <YZWUV2jvoOS9RSq8@google.com>
Date:   Wed, 17 Nov 2021 23:46:31 +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 1/4] x86/kvm: add boot parameter for adding vcpu-id
 bits

On Wed, Nov 17, 2021, Juergen Gross wrote:
> On 16.11.21 15:10, Juergen Gross wrote:
> > Today the maximum vcpu-id of a kvm guest's vcpu on x86 systems is set
> > via a #define in a header file.
> > 
> > In order to support higher vcpu-ids without generally increasing the
> > memory consumption of guests on the host (some guest structures contain
> > arrays sized by KVM_MAX_VCPU_IDS) add a boot parameter for adding some
> > bits to the vcpu-id. Additional bits are needed as the vcpu-id is
> > constructed via bit-wise concatenation of socket-id, core-id, etc.
> > As those ids maximum values are not always a power of 2, the vcpu-ids
> > are sparse.
> > 
> > The additional number of bits needed is basically the number of
> > topology levels with a non-power-of-2 maximum value, excluding the top
> > most level.
> > 
> > The default value of the new parameter will be 2 in order to support
> > today's possible topologies. The special value of -1 will use the
> > number of bits needed for a guest with the current host's topology.
> > 
> > Calculating the maximum vcpu-id dynamically requires to allocate the
> > arrays using KVM_MAX_VCPU_IDS as the size dynamically.
> > 
> > Signed-of-by: Juergen Gross <jgross@...e.com>
> 
> Just thought about vcpu-ids a little bit more.
> 
> It would be possible to replace the topology games completely by an
> arbitrary rather high vcpu-id limit (65536?) and to allocate the memory
> depending on the max vcpu-id just as needed.
> 
> Right now the only vcpu-id dependent memory is for the ioapic consisting
> of a vcpu-id indexed bitmap and a vcpu-id indexed byte array (vectors).
> 
> We could start with a minimal size when setting up an ioapic and extend
> the areas in case a new vcpu created would introduce a vcpu-id outside
> the currently allocated memory. Both arrays are protected by the ioapic
> specific lock (at least I couldn't spot any unprotected usage when
> looking briefly into the code), so reallocating those arrays shouldn't
> be hard. In case of ENOMEM the related vcpu creation would just fail.
> 
> Thoughts?

Why not have userspace state the max vcpu_id it intends to creates on a per-VM
basis?  Same end result, but doesn't require the complexity of reallocating the
I/O APIC stuff.