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

[Alexander Graf]

On 12.10.2011, at 20:05, Stefan Weil wrote:

> Hello Avi,
> 
> commit 36b58628 increased the alignment for some large memory
> blocks (typically the system RAM) to 2 MiB (QEMU_VMALLOC_ALIGN)
> on x86_64 Linux hosts.
> 
> As far as I know, this was only required for KVM.
> 
> There is a bad side effect of this increase: the Valgrind tool
> only supports an alignment of up to 1 MiB. It aborts execution
> with current QEMU for any target (even non-KVM targets).
> 
> It might be possible to modify Valgrind (as far as I know this
> is already discussed), and of course I can also patch my local
> QEMU. Nevertheless, I think the alignment should be reduced
> again when there is no KVM support or KVM is disabled.
> Maybe the large alignment has other unwanted side effects.

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