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Re: [Xen-devel] [PATCH V4 6/8] iommu/arm: Add lightweight iommu_fwspec support

Hi, all.

+struct iommu_fwspec {
+    /* this device's IOMMU */
+    struct device *iommu_dev;
+    /* IOMMU driver private data for this device */
+    void *iommu_priv;
+    /* number of associated device IDs */
+    unsigned int num_ids;
+    /* IDs which this device may present to the IOMMU */
+    uint32_t ids[1];
Note that you abuse xrealloc_flex_struct() when using it with such
a type: The last field is _not_ a flexible array member. Compilers
might legitimately warn if they can prove that you access
p->ids[1] anywhere, despite you (presumably) having allocated enough
space. (I haven't been able to think of a way for the macro to
actually detect and hence refuse such wrong uses.)
Indeed, you are right. I am in doubt, whether to retain ported from
Linux code (ids[1])

and mention about such abuse or change it to deal with real flexible
array member (ids[]). Any thoughts?
I'm of the strong opinion that you should switch to [] (or at
least [0]) notation.
I got it. Well, will switch to ids[] if there are no objections.
I suspect the rationale to use 1 rather than 0 is to avoid the re-allocation in
the common case where a device has a single ID.

I would like to retain the similar behavior. The ids[1] is probably the most
pretty way to do it.

Another solution would to use xmalloc_bytes() for the initial allocation of
Why not use xmalloc_flex_<whatever>() on the first allocation, too?
Hmm, why not? Looks like the xmalloc_flex_struct fits here.

The first allocation would be:

fwspec = xmalloc_flex_struct(struct iommu_fwspec,  ids, 1);

The re-allocation for the devices with single ID would do effectively nothing (assuming that _xrealloc will recognize that size is the same):

fwspec = xrealloc_flex_struct(fwspec, ids, fwspec->num_ids + num_ids);

Julien, what do you think?

I am happy with that. The first allocation would need a comment on top explaining the reason of the "1".

Sure, will add.


Oleksandr Tyshchenko

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