Re: [Qemu-devel] [RFC] virtio-mem: paravirtualized memory

[Stefan Hajnoczi]

On Mon, Jun 19, 2017 at 12:26:52PM +0200, David Hildenbrand wrote:
> On 19.06.2017 12:08, Stefan Hajnoczi wrote:
> > 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
> > 
> 
> arch/x86/mm/init_64.c:
> 
> /*
>  * Memory is added always to NORMAL zone. This means you will never get
>  * additional DMA/DMA32 memory.
>  */
> int arch_add_memory(int nid, u64 start, u64 size, bool for_device)
> {
> 
> The is for sure something to work on in the future. Until then, base
> memory of 3.X GB should be sufficient, right?

I'm not sure that helps because applications typically don't control
where their buffers are located?

Stefan

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