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

[Igor Mammedov]

On Mon, 25 Nov 2013 14:35:18 +0100
Paolo Bonzini <address@hidden> wrote:

> Il 25/11/2013 12:39, Gerd Hoffmann ha scritto:
> >> > 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.
> 
> Also because it's just non-PAE 32-bit guests, no?  PAE guests would
> support hotplug just fine.
Yes, it shouldn't be issue for PAE in general.

but it might not work for Windows Server 2003 (including 64-bit one), since
it BSODed when it sees 64bit CRS (we saw it playing with 64-bit PCI window).
Not sure if we care about it though. 

> Paolo
>