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

From: Alexander Graf
Subject: Re: [Qemu-arm] [PATCH 2/2] arm/virt: Mark pcie controller node as dma-coherent
Date: Thu, 2 Jun 2016 14:45:11 +0200
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0) Gecko/20100101 Thunderbird/45.1.1

On 02.06.16 14:32, Peter Maydell wrote:
> On 2 June 2016 at 13:26, Bogdan Purcareata <address@hidden> 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.


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