Re: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and check support for requests

[Logan Gunthorpe]

On 05/09/18 01:45 PM, Jens Axboe wrote:
> The point is that the caller doesn't necessarily know where the bio
> will end up, hence the caller can't fully check if the whole stack
> supports P2P.
> 
> What happens if a P2P request ends up with a driver that doesn't
> support it?

Yes, that's the whole point this check. Although we expect the caller to
do other checks before submitting a P2P request to a queue, so if a
driver does submit a P2P request to an unsupported queue, it is
definitely a problem in the driver (which is why we want to WARN).

Queues that support P2P (only PCI NVMe at this time, see patch 10) must
set QUEUE_FLAG_PCI_P2PDMA to indicate it. The check we are adding in
blk-core is meant to ensure any broken drivers that submit requests with
P2P memory do not get sent to a queue that doesn't indicate support.

On top of that, the code in NVMe target ensures that all namespaces on a
port are backed by queues that support P2P and, if not, it never
allocates any P2P SGLs.

Logan