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

[Laszlo Ersek]

On 02/09/17 19:27, Alexander Graf wrote:
> 
> 
>> Am 09.02.2017 um 19:13 schrieb Michael S. Tsirkin <address@hidden>:
>>
>>> 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 <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
>>
>> 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