On Mon, Sep 21, 2009 at 08:37:18PM -0300, Marcelo Tosatti wrote:
> Use two steps for memslot deletion: mark the slot invalid (which stops
> instantiation of new shadow pages for that slot, but allows destruction),
> then instantiate the new empty slot.
>
> Also simplifies kvm_handle_hva locking.
>
> Signed-off-by: Marcelo Tosatti <[email protected]>
>
<snip>
> - if (!npages)
> + if (!npages) {
> + slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
> + if (!slots)
> + goto out_free;
> + memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
> + if (mem->slot >= slots->nmemslots)
> + slots->nmemslots = mem->slot + 1;
> + slots->memslots[mem->slot].flags |= KVM_MEMSLOT_INVALID;
> +
> + old_memslots = kvm->memslots;
> + rcu_assign_pointer(kvm->memslots, slots);
> + synchronize_srcu(&kvm->srcu);
> + /* From this point no new shadow pages pointing to a deleted
> + * memslot will be created.
> + *
> + * validation of sp->gfn happens in:
> + * - gfn_to_hva (kvm_read_guest, gfn_to_pfn)
> + * - kvm_is_visible_gfn (mmu_check_roots)
> + */
> kvm_arch_flush_shadow(kvm);
> + kfree(old_memslots);
> + }
>
> r = kvm_arch_prepare_memory_region(kvm, &new, old, user_alloc);
> if (r)
> goto out_free;
>
> - spin_lock(&kvm->mmu_lock);
> - if (mem->slot >= kvm->memslots->nmemslots)
> - kvm->memslots->nmemslots = mem->slot + 1;
> +#ifdef CONFIG_DMAR
> + /* map the pages in iommu page table */
> + if (npages)
> + r = kvm_iommu_map_pages(kvm, &new);
> + if (r)
> + goto out_free;
> +#endif
>
> - *memslot = new;
> - spin_unlock(&kvm->mmu_lock);
> + slots = kzalloc(sizeof(struct kvm_memslots), GFP_KERNEL);
> + if (!slots)
> + goto out_free;
> + memcpy(slots, kvm->memslots, sizeof(struct kvm_memslots));
> + if (mem->slot >= slots->nmemslots)
> + slots->nmemslots = mem->slot + 1;
> +
> + /* actual memory is freed via old in kvm_free_physmem_slot below */
> + if (!npages) {
> + new.rmap = NULL;
> + new.dirty_bitmap = NULL;
> + for (i = 0; i < KVM_NR_PAGE_SIZES - 1; ++i)
> + new.lpage_info[i] = NULL;
> + }
> +
> + slots->memslots[mem->slot] = new;
> + old_memslots = kvm->memslots;
> + rcu_assign_pointer(kvm->memslots, slots);
> + synchronize_srcu(&kvm->srcu);
>
> kvm_arch_commit_memory_region(kvm, mem, old, user_alloc);
Paul,
There is a scenario where this path, which updates KVM memory slots, is
called relatively often.
Each synchronize_srcu() call takes about 10ms (avg 3ms per
synchronize_sched call), so this is hurting us.
Is this expected? Is there any possibility for synchronize_srcu()
optimization?
There are other sides we can work on, such as reducing the memory slot
updates, but i'm wondering what can be done regarding SRCU itself.
TIA
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