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 <[email protected]> >> 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 <[email protected]> >> Message-id: >> [email protected] >> Reviewed-by: Peter Maydell <[email protected]> >> Signed-off-by: Peter Maydell <[email protected]> >> >>> 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
