Messages in this thread | | | From | Juergen Gross <> | Subject | Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits | Date | Wed, 17 Nov 2021 07:59:29 +0100 |
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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?
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |