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.