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

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Message-ID: <20180901082812.GB670@lst.de>
Date:   Sat, 1 Sep 2018 10:28:12 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org,
        Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Christian König <christian.koenig@....com>
Subject: Re: [PATCH v5 07/13] block: Add PCI P2P flag for request queue and
 check support for requests

On Thu, Aug 30, 2018 at 01:11:18PM -0600, Jens Axboe wrote:
> I think this belongs in the caller - both the validity check, and
> passing in NOMERGE for this type of request. I don't want to impose
> this overhead on everything, for a pretty niche case.

It is just a single branch, which will be predicted as not taken
for non-P2P users.  The benefit is that we get proper error checking
by doing it in the block code.