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

[Laszlo Ersek]

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 <address@hidden>

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