linux-kernel - Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use of SWIOTLB bounce buffers

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Message-ID: <20250403034143-mutt-send-email-mst@kernel.org>
Date: Thu, 3 Apr 2025 03:43:31 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Christoph Hellwig <hch@...radead.org>
Cc: David Woodhouse <dwmw2@...radead.org>, virtio-comment@...ts.linux.dev,
	Claire Chang <tientzu@...omium.org>,
	linux-devicetree <devicetree@...r.kernel.org>,
	Rob Herring <robh+dt@...nel.org>,
	Jörg Roedel <joro@...tes.org>,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	graf@...zon.de
Subject: Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use
 of SWIOTLB bounce buffers

On Thu, Apr 03, 2025 at 12:39:39AM -0700, Christoph Hellwig wrote:
> On Thu, Apr 03, 2025 at 08:37:20AM +0100, David Woodhouse wrote:
> > Hm. I was just trying to point out what seemed obvious, that when a PCI
> > device does 'DMA' to an address region which is actually within one of
> > its *own* BARs,
> 
> PCIe devices can't do DMA to their own BARs by definition, see the route
> to self rule.
> 
> Pretending that they do it by parsing the addresses is bound to fail
> because the addresses seen by the driver and the device can be
> different.
> 
> NVMe got this wrong not just once but twice and is still suffering from
> this misunderstanding.  If you want to enhance a protocol to support
> addressing a local indirection buffer do not treat it as fake DMA
> but rather use explicit addressing for it, or you will be in a world of
> trouble.

Hm. This is what this proposal does, indeed. Can be changed though -
since all accesses are now within the SWIOTLB, they can be treated
as offsets instead.

-- 
MST