On Fri, Sep 10, 2021, Sean Christopherson wrote: > On Mon, Aug 09, 2021, Zeng Guang wrote: > > + if (!pages) > > + return -ENOMEM; > > + > > + to_kvm_vmx(kvm)->pid_table = (void *)page_address(pages); > > + to_kvm_vmx(kvm)->pid_last_index = KVM_MAX_VCPU_ID; > > I don't see the point of pid_last_index if we're hardcoding it to KVM_MAX_VCPU_ID. > If I understand the ucode pseudocode, there's no performance hit in the happy > case, i.e. it only guards against out-of-bounds accesses. > > And I wonder if we want to fail the build if this grows beyond an order-1 > allocation, e.g. > > BUILD_BUG_ON(PID_TABLE_ORDER > 1); > > Allocating two pages per VM isn't terrible, but 4+ starts to get painful when > considering the fact that most VMs aren't going to need more than one page. For > now I agree the simplicity of not dynamically growing the table is worth burning > a page. Ugh, Paolo has queued a series which bumps KVM_MAX_VCPU_ID to 4096[*]. That makes this an order-3 allocation, which is quite painful. One thought would be to let userspace declare the max vCPU it wants to create, not sure if that would work for xAPIC though. [*] https://lkml.kernel.org/r/1111efc8-b32f-bd50-2c0f-4c6f506b544b@xxxxxxxxxx
