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Date: Wed, 12 Oct 2011 22:41:35 +0200
From: Stefan Weil <sw@weilnetz.de>
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Subject: Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc()
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To: Alexander Graf <agraf@suse.de>
Cc: Avi Kivity <avi@redhat.com>, QEMU Developers <qemu-devel@nongnu.org>

Am 12.10.2011 22:02, schrieb Alexander Graf:
> Actually, I'd much rather prefer to keep the differences between KVM 
> and non-KVM low here. THP can potentially also work on TCG, so the 
> alignment isn't completely moot here. Though it's certainly a lot less 
> useful, as code isn't directly executed from there and we the rest of 
> the overhead is a lot higher either way (especially the softmmu one).
> Either way, why does valgrind break when we enforce big alignment? That really sounds more like a valgrind bug than anything else. Memalign is there for exactly that reason, no?
>
>
> Alex

Actually, there is even a difference between KVM (x86_64) and KVM (non 
x86_64)
in the current code: only x86_64 hosts use the 2 MiB alignment.

Valgrind breaks because it has an assertion which limits the alignment.
This limitation was already discussed in 2008 and still exists in latest
Ubuntu and other distributions (and also in latest Valgrind SVN trunk).

Therefore I don't expect that it will be fixed soon.

See these bug reports, for example:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=489297
http://bugs.kde.org/show_bug.cgi?id=203877

Cheers,
Stefan