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@xxxxxxxx> > > 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.
