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Re: [Qemu-devel] [PATCH] target-arm: Declare virtio-mmio as dma-coherent

From: Laszlo Ersek
Subject: Re: [Qemu-devel] [PATCH] target-arm: Declare virtio-mmio as dma-coherent in dt
Date: Wed, 8 Feb 2017 16:29:59 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

CC'ing Ard and Shannon (I recall this property from earlier):

On 02/08/17 14:31, Alexander Graf wrote:
> QEMU emulated hardware is always dma coherent with its guest. We do
> annotate that correctly on the PCI host controller, but left out
> virtio-mmio.

I recommend to reference the following commit here:

commit 5d636e21c44ecf982a22a7bc4ca89186079ac283
Author: Ard Biesheuvel <address@hidden>
Date:   Mon Jul 4 13:06:36 2016 +0100

    hw/arm/virt: mark the PCIe host controller as DMA coherent in the DT
    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.
    This brings the DT description into line with the ACPI description,
    which already marks the PCI bridge as cache coherent (see commit
    Signed-off-by: Ard Biesheuvel <address@hidden>
    Message-id: address@hidden
    Reviewed-by: Peter Maydell <address@hidden>
    Signed-off-by: Peter Maydell <address@hidden>

> Recent kernels have started to interpret that flag rather than take
> dma coherency as granted with virtio-mmio. While that is considered
> a kernel bug, as it breaks previously working systems, it showed that
> our dt description is incomplete.
> This patch adds the respective marker that allows guest OSs to evaluate
> that our virtio-mmio devices are indeed cache coherent.

As noted above, commit bc64b96c984a ("hw/arm/virt-acpi-build: _CCA attribute is 
compulsory", 2015-11-03) had done the same in the ACPI description of the PCIe 
host controller.

Thus, do we need _CCA in the ACPI description of the virtio-mmio transports, to 
parallel the DT change? See the LNRO0005 device in acpi_dsdt_add_virtio().

If that's the case, then I propose that either the patch please fix both DT and 
ACPI, or that at least we file a bug "somewhere", for adding _CCA in 

Ard, Shannon, any comments?


> Signed-off-by: Alexander Graf <address@hidden>
> ---
>  hw/arm/vexpress.c | 1 +
>  hw/arm/virt.c     | 1 +
>  2 files changed, 2 insertions(+)
> diff --git a/hw/arm/vexpress.c b/hw/arm/vexpress.c
> index 58760f4..e057568 100644
> --- a/hw/arm/vexpress.c
> +++ b/hw/arm/vexpress.c
> @@ -452,6 +452,7 @@ static int add_virtio_mmio_node(void *fdt, uint32_t 
> acells, uint32_t scells,
>                                         acells, addr, scells, size);
>      qemu_fdt_setprop_cells(fdt, nodename, "interrupt-parent", intc);
>      qemu_fdt_setprop_cells(fdt, nodename, "interrupts", 0, irq, 1);
> +    qemu_fdt_setprop(fdt, nodename, "dma-coherent", NULL, 0);
>      g_free(nodename);
>      if (rc) {
>          return -1;
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 1f216cf..14881fa 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -797,6 +797,7 @@ static void create_virtio_devices(const VirtMachineState 
> *vms, qemu_irq *pic)
>          qemu_fdt_setprop_cells(vms->fdt, nodename, "interrupts",
>                                 GIC_FDT_IRQ_TYPE_SPI, irq,
>                                 GIC_FDT_IRQ_FLAGS_EDGE_LO_HI);
> +        qemu_fdt_setprop(vms->fdt, nodename, "dma-coherent", NULL, 0);
>          g_free(nodename);
>      }
>  }

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