Re: [Qemu-devel] [PATCH] Permit -mem-path without sync mmu

[David Gibson <david@gibson.dropbear.id.au>]

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