Re: [Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags

["Michael S. Tsirkin" <mst@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 <lersek@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