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Re: [Xen-devel] [PATCH 1/3] AMD/IOMMU: fix off-by-one in amd_iommu_get_paging_mode() callers
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 10 Feb 2020 13:50:21 +0000
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On 05/02/2020 09:42, Jan Beulich wrote:
> amd_iommu_get_paging_mode() expects a count, not a "maximum possible"
> value. Prior to b4f042236ae0 dropping the reference, the use of our mis-
> named "max_page" in amd_iommu_domain_init() may have lead to such a
> misunderstanding.
>
> Also replace a literal 4 by an expression tying it to a wider use
> constant, just like amd_iommu_quarantine_init() does.
>
> Fixes: ea38867831da ("x86 / iommu: set up a scratch page in the quarantine
> domain")
> Fixes: b4f042236ae0 ("AMD/IOMMU: Cease using a dynamic height for the IOMMU
> pagetables")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> Note: I'm not at the same time adding error checking here, despite
> amd_iommu_get_paging_mode() possibly returning one, as I think
> that's a sufficiently orthogonal aspect.
It is entirely non-obvious what amd_iommu_get_paging_mode() takes, which
is presumably what has led to this confusion.
It also seems silly to force a call into another translation unit when
2/3 of the callers can be evaluated at compile time.
How about re-implementing amd_iommu_get_paging_mode() as a static inline
(seeing as it is just basic arithmetic), and naming its parameter in a
more useful, e.g. max_frames ?
~Andrew
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