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


From: Alexander Graf
Subject: Re: [Qemu-devel] [PATCH v2 0/4] target-arm: Add some omitted dma cache coherency flags
Date: Thu, 9 Feb 2017 19:27:15 +0100


> 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.

Alex





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