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

[Robin Murphy]

On 2024-06-12 1:50 pm, Philippe Mathieu-Daudé wrote:
> On 12/6/24 14:48, Peter Maydell wrote:
> > 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.
>
> OK. Still the question about other virt machines remains
> unanswered :)

From the DT consumer point of view, the interpretation and assumptions 
around coherency *are* generally architecture- or platform-specific. For 
instance on RISC-V, many platforms want to assume coherency by default 
(and potentially use "dma-noncoherent" to mark individual devices that 
aren't), while others may still want to do the opposite and use 
"dma-coherent" in the same manner as Arm and AArch64. Neither property 
existed back in ePAPR, so typical PowerPC systems wouldn't even be 
looking and will just make their own assumptions by other means.

Thanks,
Robin.