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

[Avi Kivity]

On 10/12/2011 10:02 PM, Alexander Graf wrote:
> 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).

Note thp will still work with tcg, even without alignment.  The
alignment requirement is specific to kvm.

> 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?
>

Agreed, seems like a valgrind limitation that needs to be lifted if
possible.

-- 
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