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Date: Fri, 17 Oct 2025 08:25:19 +0200
From: Christoph Hellwig <hch@lst.de>
To: Leon Romanovsky <leon@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>, Keith Busch <kbusch@kernel.org>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH 3/3] block-dma: properly take MMIO path
Message-ID: <20251017062519.GC402@lst.de>
References: <20251017-block-with-mmio-v1-0-3f486904db5e@nvidia.com> <20251017-block-with-mmio-v1-3-3f486904db5e@nvidia.com>
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In-Reply-To: <20251017-block-with-mmio-v1-3-3f486904db5e@nvidia.com>
User-Agent: Mutt/1.5.17 (2007-11-01)

On Fri, Oct 17, 2025 at 08:32:00AM +0300, Leon Romanovsky wrote:
> From: Leon Romanovsky <leonro@nvidia.com>
> 
> Make sure that CPU is not synced and IOMMU is configured to take
> MMIO path by providing newly introduced DMA_ATTR_MMIO attribute.

Please write a commit log that explains this.  Where was DMA_ATTR_MMIO
recently introduced?  Why?  What does this actually fix or improve?

> @@ -184,6 +184,12 @@ static bool blk_dma_map_iter_start(struct request *req, struct device *dma_dev,
>  		 * P2P transfers through the host bridge are treated the
>  		 * same as non-P2P transfers below and during unmap.
>  		 */
> +		if (iter->iter.is_integrity)
> +			bio_integrity(req->bio)->bip_flags |= BIP_MMIO;
> +		else
> +			req->cmd_flags |= REQ_MMIO;
> +		iter->iter.attrs |= DMA_ATTR_MMIO;

REQ_MMIO / BIP_MMIO is not block layer state, but driver state resulting
from the dma mapping.  Reflecting it in block layer data structures
is not a good idea.  This is really something that just needs to be
communicated outward and recorded in the driver.  For nvme I suspect
two new flags in nvme_iod_flags would be the right place, assuming
we actually need it.  But do we need it?  If REQ_/BIP_P2PDMA is set,
these are always true.