Re: [Qemu-devel] [PATCH 1/2] exec: Convert bounce buffer to a set

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/03/2015 08:50, Fam Zheng wrote:
> +    QLIST_ENTRY(BounceBuffer) next;

Where is this used?

> -    if (buffer != bounce.buffer) {
> +    BounceBuffer *bounce;
> +
> +    bounce = bounce_buffer_find_and_remove(buffer);
> +    if (!bounce) {

I'm afraid that this adds a mutex lock/unlock pair and a hash table
lookup on a very hot path.  One possibility is to add a check that the
hash table is not empty in bounce_buffer_find_and_remove.  That can be
done outside the lock so it's fast.

I'm also wondering if it's okay to let the guest do arbitrarily large
memory allocations (e.g. DMA from a huge unassigned memory area above
guest RAM); effectively you're disabling the "/* Avoid unbounded
allocations */" safety guard.

Is it hard to do this while keeping map_clients?

Paolo