Re: [Qemu-arm] [Qemu-devel] [PATCH 2/2] arm/virt: Mark pcie controller node as dma-coherent

[Alexander Graf <agraf@suse.de>]

On 06/03/2016 05:16 PM, Mihai Claudiu Caraman wrote:
>> -----Original Message-----
>> From: Peter Maydell [mailto:peter.maydell@linaro.org]
>> Sent: Friday, June 03, 2016 5:38 PM
>> To: Mihai Claudiu Caraman <mike.caraman@nxp.com>
>> Cc: Bogdan Purcareata <bogdan.purcareata@nxp.com>; QEMU Developers <qemu-devel@nongnu.org>; Peter Crosthwaite <crosthwaite.peter@gmail.com>; Alexander Graf <agraf@suse.de>; qemu-arm <qemu-arm@nongnu.org>; Eric Auger <eric.auger@linaro.org>
>> Subject: Re: [Qemu-devel] [PATCH 2/2] arm/virt: Mark pcie controller node as dma-coherent
>>
>> On 3 June 2016 at 15:22, Mihai Claudiu Caraman <mike.caraman@nxp.com> wrote:
>>> In particular for virtual and emulated devices the host CPU behaves as
>>> a DMA coherent 'device'. This should have been stated in patch
>>> description.
>> Wouldn't that imply that we should just always have the "dma-coherent"
>> property set, and we don't need to do any of the messing around looking at the host sysfs ?
>>
>> thanks
>> -- PMM
> We can always set "dma-coherent" for virtual and emulated devices but not for passthrough devices. So we can't have one PCIe controller for all devices marked as "dma-coherent".

The original patch is about the case where PCI is already cache coherent 
on the host.

I think at the end of the day this is simply outside of QEMU's scope to 
decide. What we can do is set dma-coherent per default (if Will and Ard 
agree) on the default PCIe bus and add code that allows to spawn a 
secondary PCIe bus which can then have different dma-coherent attributes 
and that you can then plug your non-coherent vfio devices into.


Alex