Re: [RFC PATCH v3 07/18] xen/arm: ITS: implement hw_irq_controller for LPIs

[Ian Campbell <ian.campbell@citrix.com>]

On Tue, 2015-06-23 at 15:32 +0100, Julien Grall wrote:
[...]
> > +{
> > +    struct its_collection *col;
> > +    struct its_device *its_dev = get_irq_device(desc);
> > +    u8 *cfg;
> > +    u32 virq = irq_to_virq(desc);
> > +
> > +    ASSERT(virq < its_dev->nr_lpis);
> > +
> > +    cfg = gic_rdists->prop_page + desc->irq - NR_GIC_LPI;
> > +    if ( enable )
> > +        *cfg |= LPI_PROP_ENABLED;
> > +    else
> > +        *cfg &= ~LPI_PROP_ENABLED;
> > +
> > +    /*
> > +     * Make the above write visible to the redistributors.
> > +     * And yes, we're flushing exactly: One. Single. Byte.
> > +     * Humpf...
> > +     */
> > +    if ( gic_rdists->flags & RDIST_FLAGS_PROPBASE_NEEDS_FLUSHING )
> > +        clean_and_invalidate_dcache_va_range(cfg, sizeof(*cfg));
> > +    else
> > +        dsb(ishst);
> > +
> > +    /* Get collection id for this event id */
> > +    col = &its_dev->its->collections[virq % num_online_cpus()];
> 
> This is fragile, you are assuming that num_online_cpus() will never
> change. Why don't you store the collection in every irq_desc?

The original Linux code upon which this is based doesn't seem to need to
lookup the collection here, why is flushing needed for us but not Linux?

I'm also confused by the use of the variable name "virq" in a function
called set_lpi_config which appears to be dealing with host level
physical LPIs. It seems like this function would be broken for LPIs
which were delivered to Xen and not to a guest, and that the irq_to_virq
in here ought to be desc->irq, no?

BTW, the Linux original calls what this calls "virq" "id" instead, which
is much less confusing.