On 02/09/17 19:27, Alexander Graf wrote: > > >> Am 09.02.2017 um 19:13 schrieb Michael S. Tsirkin <m...@redhat.com>: >> >>> On Thu, Feb 09, 2017 at 02:15:36PM +0100, Laszlo Ersek wrote: >>>> On 02/09/17 13:53, Alexander Graf wrote: >>>> ARM is amazing when it comes to cache coherency and VMs. While any sane >>>> architecture allows the host to override the guest's caching attributes, >>>> that's very hard to do on ARM. >>>> >>>> That means that the guest may directly access guest memory bypassing the >>>> cache while QEMU happily writes to / reads from cache. The end result is >>>> very nasty, because both sides see very different views of the world. >>>> >>>> That means that we need to be very cautious to tell guests that devices >>>> that QEMU emulates are going to use data in the cache rather than directly >>>> on memory. >>>> >>>> We added this to PCI a while back for DT (5d636e21 "hw/arm/virt: mark the >>>> PCIe >>>> host controller as DMA coherent in the DT") and ACPI (bc64b96 "hw/arm/virt- >>>> acpi-build: _CCA attribute is compulsory") but never updated virtio-mmio or >>>> fw-cfg in DT or ACPI tables. >>>> >>>> This patch set adds the respective cache coherency flags for them in both >>>> DT and >>>> ACPI. >>>> >>>> Fortunately, no guests except for Linux 4.9.7 and 4.9.8 are broken because >>>> of >>>> this. Upstream realized quickly enough that every user of virtio-mmio out >>>> there >>>> describes its cache coherency incorrectly and reverted the patch that would >>>> require said dma coherency flag. But we should be safe for the future and >>>> "do >>>> the right thing". >>>> >>>> Alexander Graf (4): >>>> target-arm: Declare virtio-mmio as dma-coherent in dt >>>> hw/arm/virt: Declare virtio-mmio as dma cache coherent in ACPI >>>> hw/arm/virt: Declare fwcfg as dma cache coherent in ACPI >>>> hw/arm/virt: Declare fwcfg as dma cache coherent in dt >>>> >>>> hw/arm/vexpress.c | 1 + >>>> hw/arm/virt-acpi-build.c | 2 ++ >>>> hw/arm/virt.c | 2 ++ >>>> 3 files changed, 5 insertions(+) >>>> >>> >>> Famous last words: >>> series >>> Reviewed-by: Laszlo Ersek <ler...@redhat.com> >>> >>> Should we replicate patch #3 to QEMU0002 / FWCF in >>> "hw/i386/acpi-build.c" too? Or is it that we couldn't care less about >>> _CCA on x86? :) (Can't really muster the energy right now to look it up >>> in the ACPI spec, sorry!) >>> >>> Thanks >>> Laszlo >> >> ACPI spec says: >> On platforms for which existing default cache-coherency behavior of the OS >> is not adequate, _CCA >> enables the OS to adapt to the differences >> >> So I think we don't need it on x86. > > According to acpi 6.1, x86 explicitly defaults to dma coherent if _CCA is > omitted. It's only illegal for ARM.
Incredible; a finding that, for a change, does not create more work. Thanks. Laszlo
