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. -- MST
