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

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

On Fri, Jun 14, 2024 at 03:20:33PM +0530, Manivannan Sadhasivam wrote:
> On Mon, May 20, 2024 at 09:22:54AM -0400, Michael S. Tsirkin wrote:
> > 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?
> > 
> 
> Virtio spec v1.2, sec 4.1.4 says,
> 
> "Each structure can be mapped by a Base Address register (BAR) belonging to the
> function, or accessed via the special VIRTIO_PCI_CAP_PCI_CFG field in the PCI
> configuration space.
> 
> The location of each structure is specified using a vendor-specific PCI
> capability located on the capability list in PCI configuration space of the
> device."
> 
> So this means the device has to expose the virtio structures through vendor
> specific capability isn't it?

The location of the structures within BAR is specified through
a vendor specific PCI capability, yes.

Why is that a problem?

People have been asking for alternative ways to do that,
so if you can point out what your issue is specifically,
it can be fixable.

> And only in that case, it can expose VIRTIO_F_ACCESS_PLATFORM bit for making
> use of IOMMU translation.
> 
> - Mani

virtio drivers won't even know where to access the features
without the capability.


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