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Subject: Re: [PATCH 12/16] nvme-pci: Check DMA ops when indicating support for
 PCI P2PDMA
To:     Logan Gunthorpe <logang@deltatee.com>,
        <linux-kernel@vger.kernel.org>, <linux-nvme@lists.infradead.org>,
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CC:     Stephen Bates <sbates@raithlin.com>,
        Christoph Hellwig <hch@lst.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        =?UTF-8?Q?Christian_K=c3=b6nig?= <christian.koenig@amd.com>,
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        Matthew Wilcox <willy@infradead.org>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        Jakowski Andrzej <andrzej.jakowski@intel.com>,
        Minturn Dave B <dave.b.minturn@intel.com>,
        Jason Ekstrand <jason@jlekstrand.net>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Xiong Jianxin <jianxin.xiong@intel.com>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Ira Weiny <ira.weiny@intel.com>,
        Robin Murphy <robin.murphy@arm.com>
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 <20210408170123.8788-13-logang@deltatee.com>
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From:   John Hubbard <jhubbard@nvidia.com>
Message-ID: <db17d695-eab8-9078-4878-dcc0ece94d23@nvidia.com>
Date:   Mon, 3 May 2021 17:17:48 -0700
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On 5/3/21 10:17 AM, Logan Gunthorpe wrote:
...
>>>    	blk_queue_flag_set(QUEUE_FLAG_NONROT, ns->queue);
>>> -	if (ctrl->ops->flags & NVME_F_PCI_P2PDMA)
>>> +	if (ctrl->ops->supports_pci_p2pdma &&
>>> +	    ctrl->ops->supports_pci_p2pdma(ctrl))
>>
>> This is a little excessive, as I suspected. How about providing a
>> default .supports_pci_p2pdma routine that returns false, so that
>> the op is always available (non-null)? By "default", maybe that
>> means either requiring an init_the_ops_struct() routine to be
>> used, and/or checking all the users of struct nvme_ctrl_ops.
> 
> Honestly that sounds much more messy to me than simply checking if it's
> NULL before using it (which is a common, accepted pattern for ops).

OK, it's a minor suggestion, so feel free to ignore if you prefer it
the other way, sure.

> 
>> Another idea: maybe you don't really need a bool .supports_pci_p2pdma()
>> routine at all, because the existing .flags really is about right.
>> You just need the flags to be filled in dynamically. So, do that
>> during nvme_pci setup/init time: that's when this module would call
>> dma_pci_p2pdma_supported().
> 
> If the flag is filled in dynamically, then the ops struct would have to
> be non-constant. Ops structs should be constant for security reasons.
> 

Hadn't thought about keeping ops structs constant. OK.

thanks,
-- 
John Hubbard
NVIDIA