On Fri, Jun 16, 2017 at 04:20:02PM +0200, David Hildenbrand wrote: > Important restrictions of this concept: > - Guests without a virtio-mem guest driver can't see that memory. > - We will always require some boot memory that cannot get unplugged. > Also, virtio-mem memory (as all other hotplugged memory) cannot become > DMA memory under Linux. So the boot memory also defines the amount of > DMA memory. I didn't know that hotplug memory cannot become DMA memory. Ouch. Zero-copy disk I/O with O_DIRECT and network I/O with virtio-net won't be possible. When running an application that uses O_DIRECT file I/O this probably means we now have 2 copies of pages in memory: 1. in the application and 2. in the kernel page cache. So this increases pressure on the page cache and reduces performance :(. Stefan
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