Re: [PATCH v5 11/30] ARM: GICv3 ITS: introduce device mapping

[Stefano Stabellini <sstabellini@kernel.org>]

On Thu, 6 Apr 2017, Julien Grall wrote:
> Hi Stefano,
> 
> On 06/04/17 00:41, Stefano Stabellini wrote:
> > > +    /* An Interrupt Translation Table needs to be 256-byte aligned. */
> > > +    itt_addr = _xzalloc(nr_events * hw_its->itte_size, 256);
> > > +    if ( !itt_addr )
> > > +        goto out_unlock;
> > > +
> > > +    dev = xzalloc(struct its_devices);
> > > +    if ( !dev )
> > > +        goto out_unlock;
> > > +
> > > +    /*
> > > +     * Allocate the pending_irqs for each virtual LPI. They will be put
> > > +     * into the domain's radix tree upon the guest's MAPTI command.
> > > +     */
> > 
> > Please expand the comment with a link to the xen-devel conversation
> > about allocation strategy and a TODO saying that we shouldn't need to
> > preallocate all pending_irq struct, but we do it for simplicity at the
> > moment.
> 
> It would have been useful to provide a link of the conversation you mention to
> allow us answering properly rather than loosing a day for confirmation.
> Anyway, I guess you are speaking about [1].
> [1] https://lists.xenproject.org/archives/html/xen-devel/2017-03/msg03758.html

Yes, that's right.


> I agree that we would need to take into account the memory to the guest, but
> this is a work beyond GICv3 ITS support as it is not
> supported by Xen today.
> 
> Even if you do memory accounting, this would have to be reserved in advance to
> avoid failure afterwards. Which is very similar to memory allocation...
> 
> So can you explain what you would do here?

I don't think we need a full plan written down in a comment, but I think
it is fair to say that there is room for improvement, especially given
that we are already introducing other data structures such as the host
LPI array. I would write the following:

  Allocate the pending_irqs for each virtual LPI. They will be put into
  the domain's radix tree upon the guest's MAPTI command.

  TODO. For now, allocate all pending_irqs immediately.  However,
  delayed allocation strategies are possible, see this discussion:
  https://lists.xenproject.org/archives/html/xen-devel/2017-03/msg03758.html

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