Re: [RFC PATCH V1 11/12] libxl: Insert "dma-coherent" property into virtio-mmio device node

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 3 Aug 2020, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> 
> Without "dma-coherent" property present in virtio-mmio device node,
> guest assumes it is non-coherent and making non-cacheable accesses
> to the vring when the DMA API is used for vring operations.
> But virtio-mmio device which runs at the host size is making cacheable
> accesses to vring. This all may result in a loss of coherency between
> the guest and host.
> 
> With this patch we can avoid modifying guest at all, otherwise we
> need to force VirtIO framework to not use DMA API for vring operations.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>

This should also be folded in the first patch for libxl

> ---
>  tools/libxl/libxl_arm.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/tools/libxl/libxl_arm.c b/tools/libxl/libxl_arm.c
> index 469a8b0..a68fb14 100644
> --- a/tools/libxl/libxl_arm.c
> +++ b/tools/libxl/libxl_arm.c
> @@ -726,6 +726,9 @@ static int make_virtio_mmio_node(libxl__gc *gc, void *fdt,
>      res = fdt_property_interrupts(gc, fdt, &intr, 1);
>      if (res) return res;
>  
> +    res = fdt_property(fdt, "dma-coherent", NULL, 0);
> +    if (res) return res;
> +
>      res = fdt_end_node(fdt);
>      if (res) return res;
>  
> -- 
> 2.7.4
>