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


From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH 21/27] pc: add memory hotplug 440fx machine
Date: Mon, 25 Nov 2013 14:35:18 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130923 Thunderbird/17.0.9

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.

Paolo



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