Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
To:     Bart Van Assche <bvanassche@acm.org>, linux-kernel@vger.kernel.org,
        linux-pci@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-rdma@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-block@vger.kernel.org
Cc:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Keith Busch <keith.busch@intel.com>,
        Sagi Grimberg <sagi@grimberg.me>,
        Bjorn Helgaas <bhelgaas@google.com>,
        Jason Gunthorpe <jgg@mellanox.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Dan Williams <dan.j.williams@intel.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Alex Williamson <alex.williamson@redhat.com>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
        Jens Axboe <axboe@kernel.dk>
References: <20180925162231.4354-1-logang@deltatee.com>
 <20180925162231.4354-4-logang@deltatee.com>
 <1537896805.11137.25.camel@acm.org>
From:   Logan Gunthorpe <logang@deltatee.com>
Message-ID: <18e5d3f2-c56b-645b-ac96-f0d861d12cc2@deltatee.com>
Date:   Tue, 25 Sep 2018 12:46:30 -0600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101
 Thunderbird/60.0
MIME-Version: 1.0
In-Reply-To: <1537896805.11137.25.camel@acm.org>
Content-Type: text/plain; charset=UTF-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Subject: Re: [PATCH v7 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to
 adjust the bus offset
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 2018-09-25 11:33 a.m., Bart Van Assche wrote:
>> +	/*
>> +	 * p2pdma mappings are not compatible with devices that use
>> +	 * dma_virt_ops. If the upper layers do the right thing
>> +	 * this should never happen because it will be prevented
>> +	 * by the check in pci_p2pdma_add_client()
>> +	 */
>> +	if (WARN_ON_ONCE(IS_ENABLED(CONFIG_DMA_VIRT_OPS) &&
>> +			 dev->dma_ops == &dma_virt_ops))
>> +		return 0;
> 
> Are you assuming that the compiler will optimize out the dev->dma_ops
> == &dma_virt_ops test if CONFIG_DMA_VIRT_OPS=n such that no reference to the
> dma_virt_ops symbol appears in the object file? Are you sure all compilers
> and compiler versions that are used to build the Linux kernel will do that?

Hmm, I can't say for certain but I would definitely expect any sane
compiler to do so. It's a fairly obvious optimization and the kbuild
robot is happy with it.

I just did a test compile on 4.9 (which is the earliest compiler version
I have readily available) and it worked fine. The minimum gcc the kernel
supports is 4.6 right now.

I've also done a similar test on godbolt.org[1] and can't find any
compiler version that doesn't work.

Logan


[1] https://godbolt.org/z/d5Mhee