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

[Laszlo Ersek]

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!
Laszlo