Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc()

[Alexander Graf]

On 12.10.2011, at 22:41, Stefan Weil wrote:

> 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.

Right. It might make sense to find a reasonable alignment for all archs and 
just set it to that. I vote for 16MB :).

> 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

Well, yes, my point is that it's a bug in valgrind that should be fixed. I 
don't think we should special-case QEMU because of bugs in debugging software :)


Alex