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

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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,
> 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

- Mani

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