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<p class="MsoNormal">Hello, <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’ve been experimenting with VFIO with one of our FPGA cards using a Xilinx part and XDMA IP core. It’s been smooth progress so far and I’ve had no issues with bar access and also DMA mapping/transfers to and from the card. All in all,
I’m finding it a very nice userspace driver framework. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m hoping someone can help clarify my understanding of how VFIO works for DMA in terms of coherence. I’m on a standard x86_64 Intel Xeon platform.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">In the code I see:<o:p></o:p></p>
<p class="MsoNormal"><o:p></o:p></p>
<p class="MsoNormal">VFIO<o:p></o:p></p>
<p class="MsoNormal">/*<o:p></o:p></p>
<p class="MsoNormal">* IOMMU enforces DMA cache coherence (ex. PCIe NoSnoop stripping). This<o:p></o:p></p>
<p class="MsoNormal">* capability is subject to change as groups are added or removed.<o:p></o:p></p>
<p class="MsoNormal">*/<o:p></o:p></p>
<p class="MsoNormal">#define VFIO_DMA_CC_IOMMU 4<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Which implies that IOMMU sets the mappings up as coherent…. is this understanding correct?
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m more used to having scatter gather based DMAs where you need to sync for the CPU or the device depending upon who owns/accesses the memory.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The use case I am specifically looking at is if a DMA mapping is setup through VFIO and then left open whilst data is transferred from the device to host memory and then the CPU is processing this data. The pinned/mapped data buffer is
reused repeatedly as part of a ring of buffers. It’s only at the point of closing down this application that the buffer would be unmapped in vfio.
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Is there any sync type functions or equivalents I need to be aware of in this case? Can VFIO DMA mapped memory buffers be safely used in this way?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks in advance for any information you can help with.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Derek<o:p></o:p></p>
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