On 28.06.2016 17:16, Peter Maydell wrote: > On 16 June 2016 at 14:58, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: >> On 2 June 2016 at 14:45, Alexander Graf <ag...@suse.de> 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?
I will send this patch later today. Best regards, Bogdan P. > 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 >
