Messages in this thread | | | Subject | Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits | From | Juergen Gross <> | Date | Thu, 18 Nov 2021 08:45:19 +0100 |
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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)?
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |