Re: [PATCH RFC 0/4] KVM: x86: Drastically raise KVM_USER_MEM_SLOTS limit

Maciej S. Szmigiero
2021-01-15T18:49:14Z

On 15.01.2021 14:18, Vitaly Kuznetsov wrote:
> TL;DR: any particular reason why KVM_USER_MEM_SLOTS is so low?
>
> Longer version:
>
> Current KVM_USER_MEM_SLOTS limit (509) can be a limiting factor for some
> configurations. In particular, when QEMU tries to start a Windows guest
> with Hyper-V SynIC enabled and e.g. 256 vCPUs the limit is hit as SynIC
> requires two pages per vCPU and the guest is free to pick any GFN for
> each of them, this fragments memslots as QEMU wants to have a separate
> memslot for each of these pages (which are supposed to act as 'overlay'
> pages).
>
> Memory slots are allocated dynamically in KVM when added so the only real
> limitation is 'id_to_index' array which is 'short'. We don't have any
> KVM_MEM_SLOTS_NUM/KVM_USER_MEM_SLOTS-sized statically defined arrays.
>
> We could've just raised the limit to e.g. '1021' (we have 3 private
> memslots on x86) and this should be enough for now as KVM_MAX_VCPUS is
> '288' but AFAIK there are plans to raise this limit as well.
>

I have a patch series that reworks the whole memslot thing, bringing
performance improvements across the board.
Will post it in few days, together with a new mini benchmark set.

Maciej