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Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc()
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc() |
Date: |
Sun, 16 Oct 2011 10:41:38 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0 |
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.
--
error compiling committee.c: too many arguments to function
- [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Stefan Weil, 2011/10/12
- Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Alexander Graf, 2011/10/12
- Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Stefan Weil, 2011/10/12
- Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Alexander Graf, 2011/10/12
- Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Stefan Weil, 2011/10/12
- Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Alexander Graf, 2011/10/12
- Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Gerd Hoffmann, 2011/10/13
- Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(), Paolo Bonzini, 2011/10/13
Re: [Qemu-devel] 2 MiB alignment in qemu_vmalloc(),
Avi Kivity <=