Re: [RFC] Legacy Virtio Driver with Device Has Limited Memory Access

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

On Thu, May 16, 2024 at 02:59:13PM +0200, Manivannan Sadhasivam wrote:
> On Thu, May 16, 2024 at 01:38:40PM +0900, Shunsuke Mie wrote:
> > Hi virtio folks,
> > 
> 
> You forgot to CC the actual Virtio folks. I've CCed them now.
> 
> > I'm writing to discuss finding a workaround with Virtio drivers and legacy
> > devices with limited memory access.
> > 
> > # Background
> > The Virtio specification defines a feature (VIRTIO_F_ACCESS_PLATFORM) to
> > indicate devices requiring restricted memory access or IOMMU translation. This
> > feature bit resides at position 33 in the 64-bit Features register on modern
> > interfaces. When the linux virtio driver finds the flag, the driver uses DMA
> > API that handles to use of appropriate memory.
> > 
> > # Problem
> > However, legacy devices only have a 32-bit register for the features bits.
> > Consequently, these devices cannot represent the ACCESS_PLATFORM bit. As a
> > result, legacy devices with restricted memory access cannot function
> > properly[1]. This is a legacy spec issue, but I'd like to find a workaround.
> > 
> > # Proposed Solutions
> > I know these are not ideal, but I propose the following solution.
> > Driver-side:
> >     - Implement special handling similar to xen_domain.
> > In xen_domain, linux virtio driver enables to use the DMA API.
> >     - Introduce a CONFIG option to adjust the DMA API behavior.
> > Device-side:
> > Due to indistinguishability from the guest's perspective, a device-side
> > solution might be difficult.
> > 
> > I'm open to any comments or suggestions you may have on this issue or
> > alternative approaches.
> > 
> > [1] virtio-net PCI endpoint function using PCIe Endpoint Framework,
> > https://lore.kernel.org/lkml/54ee46c3-c845-3df3-8ba0-0ee79a2acab1@igel.co.jp/t/
> > The Linux PCIe endpoint framework is used to implement the virtio-net device on
> > a legacy interface. This is necessary because of the framework and hardware
> > limitation.
> > 
> 
> We can fix the endpoint framework limitation, but the problem lies with some
> platforms where we cannot write to vendor capability registers and still have
> IOMMU.
> 
> - Mani

What are vendor capability registers and what do they have to do
with the IOMMU?


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