On Mon, May 12, 2025, James Houghton wrote: > On Thu, May 8, 2025 at 7:11 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > --- > > virt/kvm/dirty_ring.c | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c > > index e844e869e8c7..97cca0c02fd1 100644 > > --- a/virt/kvm/dirty_ring.c > > +++ b/virt/kvm/dirty_ring.c > > @@ -134,6 +134,16 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring, > > > > ring->reset_index++; > > (*nr_entries_reset)++; > > + > > + /* > > + * While the size of each ring is fixed, it's possible for the > > + * ring to be constantly re-dirtied/harvested while the reset > > + * is in-progress (the hard limit exists only to guard against > > + * wrapping the count into negative space). > > + */ > > + if (!first_round) > > + cond_resched(); > > Should we be dropping slots_lock here? Could we? Yes. Should we? Eh. I don't see any value in doing so, because in practice, it's extremely unlikely anything will be waiting on slots_lock. kvm_vm_ioctl_reset_dirty_pages() operates on all vCPUs, i.e. there won't be competing calls to reset other rings. A well-behaved userspace won't be modifying memslots or dirty logs, and won't be toggling nx_huge_pages. That leaves kvm_vm_ioctl_set_mem_attributes(), kvm_inhibit_apic_access_page(), kvm_assign_ioeventfd(), snp_launch_update(), and coalesced IO/MMIO (un)registration. Except for snp_launch_update(), those are all brutally slow paths, e.g. require SRCU synchronization and/or zapping of SPTEs. And snp_launch_update() is probably fairly slow too. And dropping slots_lock only makes any sense for non-preemptible kernels, because preemptible kernels include an equivalent check in KVM_MMU_UNLOCK(). It's also possible that dropping slots_lock in this case could be a net negative. I don't think it's likely, but I don't think it's any more or less likely that droppings slots_lock is a net positive. Without performance data to guide us, it'd be little more than a guess, and I really, really don't want to set a precedence of dropping a mutex on cond_resched() without a very strong reason for doing so. > It seems like we need to be holding slots_lock to call kvm_reset_dirty_gfn(), > but that's it. Userspace can already change the memslots after enabling the > dirty ring, so `entry->slot` can already be stale, so dropping slots_lock for > the cond_resched() seems harmless (and better than not dropping it).
