Re: [PATCH v3] hw/arm/virt: Avoid unexpected warning from Linux guest on host with Fujitsu CPUs

[Peter Maydell]

On Wed, 12 Jun 2024 at 13:33, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> Hi Zhenyu,
>
> On 12/6/24 04:05, Zhenyu Zhang wrote:
> > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > index 3c93c0c0a6..3cefac6d43 100644
> > --- a/hw/arm/virt.c
> > +++ b/hw/arm/virt.c
> > @@ -271,6 +271,17 @@ static void create_fdt(VirtMachineState *vms)
> >       qemu_fdt_setprop_cell(fdt, "/", "#size-cells", 0x2);
> >       qemu_fdt_setprop_string(fdt, "/", "model", "linux,dummy-virt");
> >
> > +    /*
> > +     * For QEMU, all DMA is coherent. Advertising this in the root node
> > +     * has two benefits:
> > +     *
> > +     * - It avoids potential bugs where we forget to mark a DMA
> > +     *   capable device as being dma-coherent
> > +     * - It avoids spurious warnings from the Linux kernel about
> > +     *   devices which can't do DMA at all
> > +     */
> > +    qemu_fdt_setprop(fdt, "/", "dma-coherent", NULL, 0);
>
> OK, but why restrict that to the Aarch64 virt machine?
> Shouldn't advertise this generically in create_device_tree()?
> Or otherwise at least in the other virt machines?

create_device_tree() creates an empty device tree, not one
with stuff in it. It seems reasonable to me for this property
on the root to be set in the same place we set other properties
of the root node.

thanks
-- PMM