Re: [patch 07/10] KVM: introduce kvm->srcu and convert kvm_set_memory_region to SRCU update

[Marcelo Tosatti <mtosatti@redhat.com>]

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 <mtosatti@redhat.com>
> 

<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