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

[Julien Grall <julien.grall@xxxxxxx>]

On 19/09/2019 11:57, Oleksandr wrote:
> Hi, all.
Hi,

> > > > > > > > +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
> > > xmalloc_bytes().
> > 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".
Cheers,

--
Julien Grall

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