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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.12.16 at 15:46, <andr2000@xxxxxxxxx> wrote:
> 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?

Possibly - looking at __gnttab_unmap_common(), are you passing
GNTMAP_device_map also to the unmap operation (together with
the frame number, or else the flag would be ignored)?

Jan

> 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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