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Date: Mon, 14 Oct 2024 13:38:09 +0200
From: Christoph Hellwig <hch@lst.de>
To: Damien Le Moal <dlemoal@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>, linux-nvme@lists.infradead.org,
	Keith Busch <kbusch@kernel.org>, Sagi Grimberg <sagi@grimberg.me>,
	Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
	Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kw@linux.com>,
	Kishon Vijay Abraham I <kishon@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
	linux-pci@vger.kernel.org,
	Rick Wertenbroek <rick.wertenbroek@gmail.com>,
	Niklas Cassel <cassel@kernel.org>
Subject: Re: [PATCH v2 4/5] PCI: endpoint: Add NVMe endpoint function driver
Message-ID: <20241014113809.GA31551@lst.de>
References: <20241011121951.90019-1-dlemoal@kernel.org> <20241011121951.90019-5-dlemoal@kernel.org> <20241014084424.GC23780@lst.de> <79e57ebb-eef7-48b1-b337-845d2ef6ff49@kernel.org>
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User-Agent: Mutt/1.5.17 (2007-11-01)

On Mon, Oct 14, 2024 at 07:41:18PM +0900, Damien Le Moal wrote:
>         |    | PCIe NVMe endpoint driver |     |
> 	|    | (Handles BAR registers,   |     |
> 	|    | doorbells, IRQs, SQs, CQs |     |
> 	|    | and DMA transfers)        |     |
>         |    +---------------------------+     |
>         |                  |                   |
>         |    +---------------------------+     |
>         |    |     NVMe fabrics host     |     |
>         |    +---------------------------+     |

That whole fabrics host here is broken.  The PCI frontend needs to
talk directly to the target code instead of doing a completely
pointless roundtrip through the block layer.

> Unless I am mistaken, if I use a PCI transport as the base for the endpoint
> driver, I would be able to connect only to a PCIe nvme device as the backend, no
> ?

No.  Any fabrisc transport (or frontend) can you any of the the
backends (file / bdev and passthrough).