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

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 18, 2021, Juergen Gross wrote:
> On 18.11.21 00:46, Sean Christopherson wrote:
> > 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@suse.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.
> > 
> 
> And if the userspace doesn't do it (like today)?

Similar to my comments in patch 4, KVM's current limits could be used as the
defaults, and any use case wanting to go beyond that would need an updated
userspace.  Exceeding those limits today doesn't work, so there's no ABI breakage
by requiring a userspace change.

Or again, this could be a Kconfig knob, though that feels a bit weird in this case.
But it might make sense if it can be tied to something in the kernel's config?