linux-kernel - Re: [PATCH] KVM: IOMMU: hva align mapping page size

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Date:	Fri, 1 Nov 2013 23:14:33 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Greg Edwards <gedwards@....com>
Cc:	kvm@...r.kernel.org, iommu@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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@....com>
> Cc: stable@...r.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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