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

From: Ard Biesheuvel
Subject: Re: [Qemu-devel] [PATCH] target-arm: Declare virtio-mmio as dma-coherent in dt
Date: Thu, 9 Feb 2017 12:31:34 +0000

On 9 February 2017 at 12:29, Alexander Graf <address@hidden> wrote:
> On 08/02/2017 17:17, Laszlo Ersek wrote:
>> On 02/08/17 17:12, Alexander Graf wrote:
>>> On 02/08/2017 04:29 PM, Laszlo Ersek wrote:
>>>> 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
>>>>      bc64b96c984abf).
>>>>           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().
>>> Yes, we should also annotate it correctly in the DSDT. Today it's not a
>>> deal breaker as Linux always assumes virtio-mmio to be dma coherent, but
>>> it would make our platform description more accurate.
>>>> 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 acpi_dsdt_add_virtio().
>>> I agree that it should happen in the same patch (set). While I don't
>>> care a lot about ACPI right now (since dt is preferred on upstream
>>> kernels), I can take a look.
>> Thank you!
> In fact, don't some other devices also suffer from this? Fw-cfg is now DMA
> capable IIUC, so that should also get a _CCA attribute for example.

Correct. Any described DMA capable device should have a _CCA attribute

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