Linked by Thom Holwerda on Sun 22nd Jul 2007 00:33 UTC, submitted by liquidat
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Member since:
2005-07-08
You make several good points. There's a fundamental trade-off between performance and isolation. The line in the sand between the kernel and userspace used to be the star of the debate, and now we have guest kernels to worry about.
A few of the shortcomings you mention can be addressed. For example, DMA and memory-mapped I/O can be emulated in userspace in much the same way as they are emulated in high-memory. Bounce-buffering strategies such as the Linux kernel's SWIOTLB service are pretty much the best-case scenario for virtualizing framebuffers and other memory apertures in software. The only way to improve on this is hardware acceleration via enhanced IOMMU functionality.
Isolation is a big deal in computer science today, and we'll no doubt see many innovations in the next decade that will allow hardware and software to manage memory protection in more sophisticated ways. Remember, some commercial UNIX systems still have a fixed segmented memory model. We're only taking the first baby steps toward flexible, high-performance memory protection. The rest will come in due time.