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Date:   Tue, 5 Jul 2022 11:32:23 -0600
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To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Christoph Hellwig <hch@lst.de>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, linux-kernel@vger.kernel.org,
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From:   Logan Gunthorpe <logang@deltatee.com>
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Subject: Re: [PATCH v7 20/21] PCI/P2PDMA: Introduce pci_mmap_p2pmem()
Precedence: bulk


On 2022-07-05 11:21, Greg Kroah-Hartman wrote:
> On Tue, Jul 05, 2022 at 06:50:39PM +0200, Christoph Hellwig wrote:
>> [note for the newcomers, this is about allowing mmap()ing the PCIe
>> P2P memory from the generic PCI P2P code through sysfs, and more
>> importantly how to revoke it on device removal]
> 
> We allow mmap on PCIe config space today, right?  Why is this different
> from what pci_create_legacy_files() does today?
> 
>> On Tue, Jul 05, 2022 at 10:44:49AM -0600, Logan Gunthorpe wrote:
>>> We might be able to. I'm not sure. I'll have to figure out how to find
>>> that inode from the p2pdma code. I haven't found an obvious interface to
>>> do that.
>>
>> I think the right way to approach this would be a new sysfs API
>> that internally calls unmap_mapping_range internally instead of
>> exposing the inode. I suspect that might actually be the right thing
>> to do for iomem_inode as well.
> 
> Why do we need something new and how is this any different from the PCI
> binary files I mention above?  We have supported PCI hotplug for a very
> long time, do the current PCI binary sysfs files not work properly with
> mmap and removing a device?

The P2PDMA code allocates and hands out struct pages to userspace that
are backed with ZONE_DEVICE memory from a device's BAR. This is quite
different from the existing binary files mentioned above which neither
support struct pages nor allocation.

Logan