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Re: [Qemu-devel] [PATCH] Permit -mem-path without sync mmu
From: |
David Gibson |
Subject: |
Re: [Qemu-devel] [PATCH] Permit -mem-path without sync mmu |
Date: |
Thu, 11 Aug 2011 16:09:59 +1000 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Aug 10, 2011 at 12:01:04PM +0300, Avi Kivity wrote:
> On 08/10/2011 08:10 AM, David Gibson wrote:
> >On Mon, Aug 08, 2011 at 11:24:09AM +0300, Avi Kivity wrote:
> >> On 08/08/2011 09:03 AM, David Gibson wrote:
[snip]
> >This would seem to be a genuine bug in the hugepage code, which has
> >just been hidden by SYNC_MMU. It should be quite easy to fix - the
> >mapping is only stored in the struct page to get to the hugetlbfs
> >superblock, so we could just store a direct superblock pointer
> >instead, and bump it's refcount when we put that in the page private
> >pointer.
> >
> >But then I'm not sure how qemu would detect that it's on a kernel
> >where the bug is fixed and allow -mem-path to be used again. Any
> >ideas?
>
> If it's just a kernel bug, the fix belongs in the kernel, not in qemu.
Obviously.
> We used to have KVM_CAPs to declare this sort of thing
> (KVM_CAP_HUGETLBFS_WORKS_EVEN_WITHOUT_SYNC_MMU) but I don't think it
> was a good idea.
I tend to agree - especially since there's nothing actually kvm
specific about this bug. AFAICT a driver which did gup on hugepages
could trigger the bug equally well.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson