linux-kernel - Re: [PATCH 3/3] block-dma: properly take MMIO path

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Message-ID: <20251020085648.GN6199@unreal>
Date: Mon, 20 Oct 2025 11:56:48 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Christoph Hellwig <hch@....de>
Cc: Jens Axboe <axboe@...nel.dk>, Keith Busch <kbusch@...nel.org>,
	Sagi Grimberg <sagi@...mberg.me>, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH 3/3] block-dma: properly take MMIO path

On Fri, Oct 17, 2025 at 08:25:19AM +0200, Christoph Hellwig wrote:
> On Fri, Oct 17, 2025 at 08:32:00AM +0300, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> > 
> > Make sure that CPU is not synced and IOMMU is configured to take
> > MMIO path by providing newly introduced DMA_ATTR_MMIO attribute.

<...>

> > +		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.

We have three different flows.
1. Regular one, backed by struct page, e.g. dma_map_page()
2. PCI_P2PDMA_MAP_BUS_ADDR - non-DMA flow
3. PCI_P2PDMA_MAP_THRU_HOST_BRIDGE - DMA without struct page, e.g. dma_map_resource()

There is a need for two bits to represent them.

Thanks