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

Vitaly Kuznetsov
2021-01-15T13:21:30Z

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.

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