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

[Fam Zheng <famz@redhat.com>]

On Tue, 03/10 12:14, Paolo Bonzini wrote:
> 
> 
> On 10/03/2015 08:50, Fam Zheng wrote:
> > +    QLIST_ENTRY(BounceBuffer) next;
> 
> Where is this used?

Unused, I will remove this.

> 
> > -    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.

This is a good point, it is dangerous to allow that.

> 
> Is it hard to do this while keeping map_clients?

We might need to keep map_client for above reason. I'll take another look into
it.

Fam