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

[Mihai Claudiu Caraman]

-----Original Message-----
From: Alexander Graf [mailto:address@hidden 
Sent: Friday, June 03, 2016 6:26 PM
To: Mihai Claudiu Caraman <address@hidden>; Peter Maydell <address@hidden>
> Cc: Bogdan Purcareata <address@hidden>; QEMU Developers <address@hidden>; 
> Peter Crosthwaite <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 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. 

A dma-coherent default PCIe bus looks fine.

Thanks,
Mike