Re: [Qemu-devel] [PATCH] hw/arm/virt: mark the PCIe host controller as DMA coherent in the DT

[Ard Biesheuvel]

On 1 July 2016 at 13:40, Andrew Jones <address@hidden> wrote:
> On Thu, Jun 30, 2016 at 03:09:53PM +0100, Peter Maydell wrote:
>> On 28 June 2016 at 18:14, Ard Biesheuvel <address@hidden> wrote:
>> > Since QEMU performs cacheable accesses to guest memory when doing DMA
>> > as part of the implementation of emulated PCI devices, guest drivers
>> > should use cacheable accesses as well when running under KVM. Since this
>> > essentially means that emulated PCI devices are DMA coherent, set the
>> > 'dma-coherent' DT property on the PCIe host controller DT node.
>> >
>> > Signed-off-by: Ard Biesheuvel <address@hidden>
>> > ---
>> >  hw/arm/virt.c | 1 +
>> >  1 file changed, 1 insertion(+)
>> >
>> > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
>> > index c5c125e9204a..6e098afd1fe5 100644
>> > --- a/hw/arm/virt.c
>> > +++ b/hw/arm/virt.c
>> > @@ -1021,6 +1021,7 @@ static void create_pcie(const VirtBoardInfo *vbi, 
>> > qemu_irq *pic,
>> >      qemu_fdt_setprop_cell(vbi->fdt, nodename, "#size-cells", 2);
>> >      qemu_fdt_setprop_cells(vbi->fdt, nodename, "bus-range", 0,
>> >                             nr_pcie_buses - 1);
>> > +    qemu_fdt_setprop(vbi->fdt, nodename, "dma-coherent", NULL, 0);
>> >
>> >      if (vbi->v2m_phandle) {
>> >          qemu_fdt_setprop_cells(vbi->fdt, nodename, "msi-parent",
>> > --
>> > 2.7.4
>>
>> Applied, thanks.
>>
>> -- PMM
>>
>
> I might have mentioned in the commit message that the ACPI generation
> already does this, as _CCA is set to 1, added with commit bc64b96c
> (assuming I'm right, and a value of 1 there is the ACPI equivalent of
>  this patch)
>
> bc64b96c's commit message is also lacking, in the fact it doesn't
> state why the value of 1 is chosen, only that the attribute is
> compulsory, which I presume could have been added with the value 0
> to satisfy that.
>
> Anyway, I just wanted to point out that I *think* we're fine for
> ACPI. Perhaps I'm the only one who didn't know that already though...
>

I did a quick test to compare:

(a) mach-virt in DT mode without this patch
(b) mach-virt in DT mode with this patch
(c) mach-virt in ACPI mode (with bc64b96c applied)

with '-device nec-usb-xhci -device usb-kbd' added to the QEMU command line

(b) and (c) detect the keyboard, whereas (a) does not. So I guess that
solves the USB mystery (but perhaps Alex could confirm?)