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

[Peter Maydell]

On 16 June 2016 at 14:58, Ard Biesheuvel <address@hidden> wrote:
> On 2 June 2016 at 14:45, Alexander Graf <address@hidden> wrote:
>> On 02.06.16 14:32, Peter Maydell wrote:
>>> This patch seems to change the property of the emulated PCIe controller
>>> based on the host PCIe controller even if we're not doing any PCIe
>>> passthrough at all. That seems definitely wrong to me.
>>>
>>> (Should the purely-emulated case be marked DMA-coherent anyway?
>>> I forget the fiddly details...)
>>
>> I do too, let's involve a few people who know :). Not exposing it as
>> coherent is definitely wrong, but whether "dma-coherent" is the right
>> choice I don't know.

> As far as I understand it, the purely emulated case should be marked
> DMA coherent, since otherwise, guest drivers may perform cache
> maintenance that the host is not expecting. This is especially harmful
> if the guest invalidates the caches after a device to memory transfer,
> which may result in data being lost if the data was only present in
> the caches to begin with (which is the case for devices that are
> emulated by the host)

So the consensus seems to be that:
 * emulated PCI devices definitely need dma-coherent
 * passthrough devices where the host controller is dma-coherent
   also need dma-coherent
 * passthrough devices where the host controller is not dma-coherent
   don't want dma-coherent, but we have to set things per-PCI-controller

Would somebody like to write a patch which just unconditionally
sets the dma-coherent property on our PCI controller dt node?
That seems a clear improvement on what we have at the moment.
We can look at whether we want to support passthrough from a
non-dma-coherent host pci controller (via a 2nd guest pci controller?)
later...

thanks
-- PMM