Marly Salazar

Staff Software Engineer

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Passing a TPU Through to a Paravirtualized Guest A customer asked us to run Google TPUs inside our zones. The obvious way to hand a guest a piece of PCI hardware is IOMMU-backed passthrough, and Xen has supported that for HVM guests for years. Our zones default to PV, without hardware virtualization extensions or a VT-x or AMD-V dependency. That is a big part of why they boot and fork as fast as they do. Moving these workloads to a different guest model would have meant giving up properties we deliberately built the runtime around, so the interesting problem was making passthrough work without changing the execution model. Xen's PV mode had never had an IOMMU story for a domU. Every DMA a PV guest issues goes out under dom0's address space, because PV was designed before device passthrough to an unprivileged domain was something anyone needed from it. The TPU work ended up being three problems stacked on top of each other. We had to give Xen an IOMMU interface a PV guest could actually

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Passing a TPU Through to a Paravirtualized Guest

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