On 02 Jun 2015, at 17:07, Wei Liu [off-list ref] wrote:

On Fri, May 22, 2015 at 10:25:10AM +0000, Joao Martins wrote:

quoted

On 19 May 2015, at 17:32, Wei Liu [off-list ref] wrote:

quoted

On Tue, May 12, 2015 at 07:18:28PM +0200, Joao Martins wrote:

quoted

It starts by doing a lookup in the tree for a gref. If no persistent
grant is found on the tree, it will do grant copy and prepare
the grant maps. Finally valides the grant map and adds it to the tree.

validates?

quoted

After mapped these grants can be pulled from the tree in the subsequent
requests. If it's out of pages in the tree pool, it will fallback to
grant copy.

Again, this looks complicated. Why use combined scheme? I will do
detailed reviews after we're sure we need such scheme.

When we don't have the gref in tree we need to map it and then copying
afterwards into the newly mapped page (and this only happens once until
the grant is in tree). My options here were to either do this explicitly,
after we add the persistent grant in which we would need to save to
dst/src address and len to copy. The other option is to reuse the grant
copy (since it's only once until the grant is in the tree) and use memcpy
in followings requests. Additionally I allow the fallback to grant copy in

Which approach were you using here? I looked at the code but couldn't
quite get which one you were getting at. I guess the first one?

The one I used was the second one i.e. grant copy when the gref is not in tree,
and memcpy on subsequent requests. The only difference in these options is really
to memcpy (first option) or to grant copy (second option) on the first lookup
of the gref in the tree. The problem with the first option is adding more
state to save where to memcpy which we can only do after mapping the grant. Thus
reusing the grant copy simplifies, but probably makes things not as clear.

Perhaps what I suggested in an earlier comment (regarding TX persistent grants) could
simplify things.

quoted

case the guest provides providing more grefs > max_grants.

Note that this is also the case for TX as well, with regard to grant
copying the header. I was unsure about which one is the most correct way
of doing it, but ultimately the latter involved a smaller codepath, and
that's why I chose it. What do you think?

Shorter is better. Easier to understand.