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

[Christoph Hellwig]

On Fri, Apr 04, 2025 at 09:16:44AM +0100, David Woodhouse wrote:
> The sad part is that the system does it all automatically *if* it has
> CONFIG_DMA_RESTRICTED_POOL (e.g. Linux) and the driver never even
> notices that the dma_ops it's using are the swiotlb ops using the
> provided buffer.
> 
> Which is *kind* of nice... except that when on a guest OS which *isn't*
> Linux with CONFIG_DMA_RESTRICTED_POOL, the guest will just ignore the
> `restricted-dma-pool` node and try DMA to system memory anyway, which
> will fail.
> 
> That's why my proposal adds the negotiated VIRTIO_F_SWIOTLB feature, so
> that the device side can refuse, if the guest *isn't* agreeing to use
> the bounce buffer in the situations where it must do so.

That all assumes the indirect DMA is a device "feature".  It's not.
It is a limitation placed on the guest system.