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


From: Gerd Hoffmann
Subject: Re: [Qemu-devel] [PATCH 21/27] pc: add memory hotplug 440fx machine
Date: Mon, 25 Nov 2013 12:39:05 +0100

> > 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.

cheers,
  Gerd





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