Re: [RFC] virtio-mem: paravirtualized memory

[Stefan Hajnoczi <stefanha@gmail.com>]

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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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