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

From: Juergen Gross
Date: Wed Nov 17 2021 - 01:59:43 EST

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.



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