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

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 17/09/2019 19:18, Oleksandr wrote:
> On 17.09.19 09:12, Jan Beulich wrote:
>
> Hi, Jan
>
> > On 16.09.2019 20:08, Oleksandr wrote:
> > > On 16.09.19 13:40, Jan Beulich wrote:
> > > > > +/* per-device IOMMU instance data */
> > > > > +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().
Cheers,

--
Julien Grall

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