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

[David Hildenbrand]

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?

-- 

Thanks,

David