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Re: [Xen-devel] [PATCH 3/3] AMD/IOMMU: replace a few literal numbers

On 10.02.2020 15:28, Andrew Cooper wrote:
> On 05/02/2020 09:43, Jan Beulich wrote:
>> Introduce IOMMU_PDE_NEXT_LEVEL_{MIN,MAX} to replace literal 1, 6, and 7
>> instances. While doing so replace two uses of memset() by initializers.
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> This does not look to be an improvement.  IOMMU_PDE_NEXT_LEVEL_MIN is
> definitely bogus, and in all cases, a literal 1 is better, because that
> is how we describe pagetable levels.

I disagree. The device table entry's mode field is bounded by 1
(min) and 6 (max) for the legitimate values to put there.

> Something to replace literal 6/7 probably is ok, but doesn't want to be
> done like this.
> The majority of the problems here as caused by iommu_pde_from_dfn()'s
> silly ABI.  The pt_mfn[] array is problematic (because it is used as a
> 1-based array, not 0-based) and useless because both callers only want
> the 4k-equivelent mfn.  Fixing the ABI gets rid of quite a lot of wasted
> stack space, every use of '1', and every upper bound other than the bug
> on and amd_iommu_get_paging_mode().

I didn't mean to alter that function's behavior, at the very least
not until being certain there wasn't a reason it was coded with this
array approach. IOW the alternative to going with this patch
(subject to corrections of course) is for me to drop it altogether,
keeping the hard-coded numbers in place. Just let me know.

>> ---
>> TBD: We should really honor the hats field of union
>>      amd_iommu_ext_features, but the specification (or at least the
>>      parts I did look at in the course of putting together this patch)
>>      is unclear about the maximum valid value in case EFRSup is clear.
> It is available from PCI config space (Misc0 register, cap+0x10) even on
> first gen IOMMUs,

I don't think any of the address size fields there matches what
HATS is about (limiting of the values valid to put in a DTE's
mode field). In fact I'm having some difficulty bringing the
two in (sensible) sync.

> and the IVRS table in Type 10.

Which may in turn be absent, i.e. the question of what to use as
a default merely gets shifted.


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