> 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. Alex
