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SubjectRe: [PATCH] KVM: IOMMU: hva align mapping page size
On Fri, Nov 01, 2013 at 10:08:55AM -0600, Greg Edwards wrote:
> When determining the page size we could use to map with the IOMMU, the
> page size should be aligned with the hva, not the gfn. The gfn may not
> reflect the real alignment within the hugetlbfs file.
>
> Most of the time, this works fine. However, if the hugetlbfs file is
> backed by non-contiguous huge pages, a multi-huge page memslot starts at
> an unaligned offset within the hugetlbfs file, and the gfn is aligned
> with respect to the huge page size, kvm_host_page_size() will return the
> huge page size and we will use that to map with the IOMMU.
>
> When we later unpin that same memslot, the IOMMU returns the unmap size
> as the huge page size, and we happily unpin that many pfns in
> monotonically increasing order, not realizing we are spanning
> non-contiguous huge pages and partially unpin the wrong huge page.
>
> Instead, ensure the IOMMU mapping page size is aligned with the hva
> corresponding to the gfn, which does reflect the alignment within the
> hugetlbfs file.
>
> Signed-off-by: Greg Edwards <gedwards@ddn.com>
> Cc: stable@vger.kernel.org
> ---
> This resolves the bug previously reported (and misdiagnosed) here:
>
> http://www.spinics.net/lists/kvm/msg97599.html
>
> virt/kvm/iommu.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/virt/kvm/iommu.c b/virt/kvm/iommu.c
> index 72a130b..0e2ff32 100644
> --- a/virt/kvm/iommu.c
> +++ b/virt/kvm/iommu.c
> @@ -99,8 +99,8 @@ int kvm_iommu_map_pages(struct kvm *kvm, struct kvm_memory_slot *slot)
> while ((gfn + (page_size >> PAGE_SHIFT)) > end_gfn)
> page_size >>= 1;
>
> - /* Make sure gfn is aligned to the page size we want to map */
> - while ((gfn << PAGE_SHIFT) & (page_size - 1))
> + /* Make sure hva is aligned to the page size we want to map */
> + while (__gfn_to_hva_memslot(slot, gfn) & (page_size - 1))
> page_size >>= 1;

gfn should be aligned to page size as well (IOMMU requirement), so don't
drop that check.



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