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Date: Wed, 17 Mar 2010 16:55:42 +0100
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [Qemu-devel] [PATCH QEMU] Transparent Hugepage Support #3
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To: Paul Brook <paul@codesourcery.com>
Cc: qemu-devel@nongnu.org, Avi Kivity <avi@redhat.com>

On Wed, Mar 17, 2010 at 03:52:15PM +0000, Paul Brook wrote:
> > > > Size not multiple I think is legitimate, the below-4G chunk isn't
> > > > required to end 2M aligned, all it matters is that the above-4G then
> > > > starts aligned. In short one thing to add in the future as parameter
> > > > to qemu_ram_alloc is the physical address that the host virtual
> > > > address corresponds to.
> > >
> > > In general you don't know this at allocation time.
> > 
> > Caller knows it, it's not like the caller is outside of qemu, it's not
> > some library. We know this is enough with the caller that there is now.
> 
> No we don't.  As discussed previously, there are machines where the physical 
> location of RAM is configurable at runtime.  In fact it's common for the ram 
> to be completely absent at reset.

This is why PREFERRED_RAM_ALIGN is only defined for __x86_64__. I'm
not talking about other archs that may never support transparent
hugepages in the kernel because of other architectural constrains that
may prevent to map hugepages mixed with regular pages in the same vma.