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

["Michael S. Tsirkin" <mst@redhat.com>]

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