I'm interested too.

Muli Ben-Yehuda wrote:
> If there's interest, I'll be happy to give this talk I'll be giving at
> OLS '07 at Haifux as well.
> The Price of Safety: Evaluating IOMMU Performance
> IOMMUs, "IO Memory Management Units", are hardware devices that
> translate device DMA addresses to machine addresses. Isolation capable
> IOMMUs perform a valuable system service, preventing rogue devices
> from performing errant or malicious DMAs, thereby substantially
> increasing the system's reliability and availability. Without an
> IOMMU, a peripheral device could be programmed to overwrite any part
> of the system's memory. An isolation capable IOMMU restricts a device
> so that it can only access parts of memory it has been explicitly
> granted access to. Operating systems utilize IOMMUs to isolate device
> drivers; hypervisors utilize IOMMUs to grant secure direct hardware
> access to virtual machines. With the imminent publication of the
> PCI-SIG's IO Virtualization standard, as well as Intel and AMD's
> introduction of isolation capable IOMMUs in all new servers, IOMMUs
> will become ubiquitous.
> IOMMUs can impose a performance penalty due to the extra memory
> accesses required to perform DMA operations. The exact performance
> degradation depends on the IOMMU design, its caching architecture, the
> way it is programmed and the workload. In this paper, we present the
> performance characteristics of the Calgary and DART IOMMUs in Linux,
> both on bare metal and hypervisors. We measure the throughput and CPU
> utilization of several IO workloads with and without an IOMMU and
> analyze the results. We then discuss potential strategies for
> mitigating the IOMMU's costs. We conclude by presenting a set of
> optimizations we have implemented and the resulting performance
> improvements.
> Cheers,
> Muli
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