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

[Alexander Graf]

On 06/03/2016 05:16 PM, Mihai Claudiu Caraman wrote:
> > -----Original Message-----
> > From: Peter Maydell [mailto:address@hidden
> > Sent: Friday, June 03, 2016 5:38 PM
> > To: Mihai Claudiu Caraman <address@hidden>
> > Cc: Bogdan Purcareata <address@hidden>; QEMU Developers <address@hidden>; Peter Crosthwaite 
> > <address@hidden>; Alexander Graf <address@hidden>; qemu-arm <address@hidden>; Eric Auger 
> > <address@hidden>
> > 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 <address@hidden> 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