"Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx> writes: > Hi Vitaly, > > On 27.01.2021 18:55, Vitaly Kuznetsov wrote: >> "Maciej S. Szmigiero" <maciej.szmigiero@xxxxxxxxxx> writes: >> >>> 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. >> >> I'm about to send a successor of this series. It will be implmenting >> Sean's idea to make the maximum number of memslots a per-VM thing (and >> also raise the default). Hope it won't interfere with your work! > > Thanks for your series and CC'ing me on it. > > It looks like there should be no design conflicts, I will merely need to > rebase on top of it. > > By the way I had to change a bit the KVM selftest framework memslot > handling for my stuff, too, since otherwise just adding 32k memslots > for a test would take almost forever. > Yea, that's why PATCH5 of the new series increases the default timeout from 45 seconds to 120. -- Vitaly
