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 >
