Re: [Qemu-devel] [PATCH 21/27] pc: add memory hotplug 440fx machine

[Igor Mammedov]

On Mon, 25 Nov 2013 12:39:05 +0100
Gerd Hoffmann <address@hidden> wrote:

> > > Does it make sense to hotplug memory above 4g unconditionally to
> > > simplify things?
> > It does and it was so in v6 RFC,
> > But it would rule out hotplug for 32-bit guests that doesn't support
> > more then 4Gb.
> 
> Indeed.
> 
> > As use case 32-bit guest could start whit small initial memory
> > and hotplug additional memory if needed up to point where 32-bit
> > PCI hole starts. That would allow guests to launch with small amount
> > but baloon up upto 2-3.5 Gb depending on machine type.
> > I could drop 32-bit guest support and do only high mem hotplug if
> > this case it not interesting to the comunity, any suggestions?
> 
> 32bit limits start to hurt with 1GB already.  Kernel address space is 1G
> on 32bit, so the kernel can't map all RAM all the time any more.  Which
> in turn adds overhead for mapping/unmapping pages if the kernel must
> access highmem pages.  So it's better to run 64bit guests even with alot
> less than 4G of memory.
> 
> I'd tend to just not support 32bit guests, I think it simply isn't worth
> the trouble.
Fine with me, If there isn't abjection to it, I'll drop 32-bit guest support
on the next respin.

> cheers,
>   Gerd
> 
>