Re: [Qemu-devel] [PATCH] xen_disk: fix unmapping of persistent grants

["Roger Pau Monné" <roger.pau@citrix.com>]

El 12/11/14 a les 18.41, Stefano Stabellini ha escrit:
> On Wed, 12 Nov 2014, Roger Pau Monne wrote:
>> This patch fixes two issues with persistent grants and the disk PV backend
>> (Qdisk):
>>
>>  - Don't use batch mappings when using persistent grants, doing so prevents
>>    unmapping single grants (the whole area has to be unmapped at once).
> 
> The real issue is that destroy_grant cannot work with batch_maps.
> One could reimplement destroy_grant to build a single array with all the
> grants to unmap and make a single xc_gnttab_munmap call.
> 
> Do you think that would be feasible?

Making destroy_grant work with batch maps using the current tree
structure is going to be quite complicated, because destroy_grant
iterates on every entry on the tree, and doesn't know which grants
belong to which regions.

IMHO a simpler solution would be to introduce another tree (or list)
that keeps track of grant-mapped regions, and on tear down use the data
in that list to unmap the regions. This way the current tree will still
be used to perform the grant_ref->vaddr translation, but on teardown the
newly introduced list would be used instead.

In general I was reluctant to do this because not using batch maps with
persistent grants should not introduce a noticeable performance
regression due to the fact that grants are only mapped once for the
whole life-cycle of the virtual disk. Also, if we plan to implement
indirect descriptors for Qdisk we really need to be able to unmap single
grants in order to purge the list, since in that case it's not possible
to keep all possible grants persistently mapped.

Since this alternate solution is easy to implement I will send a new
patch using this approach, then we can decide what to do.

Roger.