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
