[Xen-devel] gnttab_end_foreign_access/unmap fails for GNTMAP_device_map?

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

Hi, all!

I am trying to map and then unmap grant refs in Dom0 from DomU
which are used for DMA by a real device in Dom0 (x86_64).
Mapping is done with GNTMAP_host_map | GNTMAP_device_map flags
and device can successfully access the pages and do DMA.
The problem is I cannot unmap the buffer back: Dom0 says
unmapping was done w/o errors, but when DomU tries to
gnttab_end_foreign_access I see lots of
[   55.052979] xen:grant_table: WARNING: g.e. 0xdce still in use!
[   55.052984] deferring g.e. 0xdce (pfn 0xffffffffffffffff)
messages in DomU and those grefs are never freed and system gets
exhausted at some point.

The same code, but with only GNTMAP_host_map flag is able to release
references as expected. Even if I don't give the buffer to any
HW device in Dom0 the use of GNTMAP_device_map flag makes troubles.
I also tried to add GNTMAP_readonly, but it didn't help.
In DomU right before doing gnttab_end_foreign_access I am checking
flags with gnttab_query_foreign_access: both GTF_reading
and GTF_writing are set. With GNTMAP_readonly only GTF_reading
(which I believe is expected).
I tried to look at the Xen sources and thought that it could
probably be related to pin/unpin?
Can anyone please shed some light if anything additional needs to
be done in case GNTMAP_device_map is in use or point me to
what needs to checked so I can find the root cause?

Thank you,
Oleksandr


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