linux-kernel - Re: [PATCH v7 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to adjust the bus offset

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Date:   Tue, 25 Sep 2018 12:46:30 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Bart Van Assche <bvanassche@....org>, 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
Cc:     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>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v7 03/13] PCI/P2PDMA: Add PCI p2pmem DMA mappings to
 adjust the bus offset



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