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

[Sean Christopherson <seanjc@google.com>]

On Fri, Jan 15, 2021, Vitaly Kuznetsov wrote:
> TL;DR: any particular reason why KVM_USER_MEM_SLOTS is so low?

Because memslots were allocated statically up until fairly recently (v5.7), and
IIRC consumed ~92kb.  Doubling that for every VM would be quite painful. 

> 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).

What exactly does QEMU do on the backend?  I poked around the code a bit, but
didn't see anything relevant.

> 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.
> 
> Vitaly Kuznetsov (4):
>   KVM: x86: Drop redundant KVM_MEM_SLOTS_NUM definition
>   KVM: mips: Drop KVM_PRIVATE_MEM_SLOTS definition
>   KVM: Define KVM_USER_MEM_SLOTS in arch-neutral
>     include/linux/kvm_host.h
>   KVM: x86: Stop limiting KVM_USER_MEM_SLOTS
> 
>  arch/mips/include/asm/kvm_host.h | 2 --
>  arch/x86/include/asm/kvm_host.h  | 3 +--
>  include/linux/kvm_host.h         | 4 ++++
>  3 files changed, 5 insertions(+), 4 deletions(-)
> 
> -- 
> 2.29.2
>