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

[Auger Eric <eric.auger@redhat.com>]

Hi,
Le 02/06/2016 à 14:45, Alexander Graf a écrit :
> 
> 
> On 02.06.16 14:32, Peter Maydell wrote:
>> On 2 June 2016 at 13:26, Bogdan Purcareata <bogdan.purcareata@nxp.com> wrote:
>>> A PCI device is marked either as coherent or non-coherent based on the pcie
>>> controller "dma-coherent" property. This is further used when configuring the
>>> IOMMU ops for the device DMA resources (e.g. descriptor rings, for e1000e).
>>>
>>> This dma-coherent property needs to be configured in the guest environment,
>>> in case there's a directly assigned VFIO PCI device. Since the guest only
>>> receives one emulated pcie controller bus - regardless of the host
>>> configuration - add this property if there's at least one host pcie host
>>> controller that is DMA coherent (this implies that the host interconnect
>>> is coherent as well).
>>
>> 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.

For information, on AMD overdrive the PCI host controller is marked as
dma-coherent too but I do not observe any issue with 82574L (e1000e)
and it worked fine as well on X540T1 Intel NIC. I just tested with
e1000e and dma-coherent guest PCI controller and works fine too. Don't
have x540t1 anymore so can't test with that PCIe CARD.

With Intel I35OT1 and guest non dma-coherent PCIe controller I cope with
some issues when assigning the PF but I don't know yet if there is any
relationship.

Best Regards

Eric
> 
> 
> Alex
>