Re: Xen/arm: Virtual ITS command queue handling

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

On Fri, 2015-05-15 at 18:17 +0530, Vijay Kilari wrote:
> On Fri, May 15, 2015 at 5:33 PM, Julien Grall <julien.grall@citrix.com> wrote:
> > On 15/05/15 12:30, Ian Campbell wrote:
> >>> Handling of Single vITS and multipl pITS can be made simple.
> >>>
> >>> All ITS commands except SYNC & INVALL has device id which will
> >>> help us to know to which pITS it should be sent.
> >>>
> >>> SYNC & INVALL can be dropped by Xen on Guest request
> >>>  and let Xen append where ever SYNC & INVALL is required.
> >>> (Ex; Linux driver adds SYNC for required commands).
> >>> With this assumption, all ITS commands are mapped to pITS
> >>> and no need of synchronization across pITS
> >>
> >> You've ignored the second bullet its three sub-bullets, I think.
> >
>    Why can't we group the batch of commands based on pITS it has
> to be sent?.

Are you suggesting that each batch we send should be synchronous? (i.e.
end with SYNC+INT) That doesn't seem at all desirable.

> > Aside ignoring the second bullet it's not possible to drop like that a
> > SYNC/INVALL command sent be the guest. How can you decide when a SYNC is
> > required or not? Why dropping "optional" SYNC would be fine? The spec
> > only says "This command specifies that all actions for the specified
> > re-distributor must be completed"...
> 
>  If Xen is sending SYNC/INVALL commands to pITS based on the commands
> Xen is sending on pITS, there is no harm in ignoring guest commands.
> 
> SYNC/INVALL are always depends on previous ITS commands.
> IMO, Alone these commands does not have any significance.
> 
> >
> > Linux is not a good example for respecting the spec. Developers may
> > decide to put SYNC differently in new necessary place and we won't be
> > able to handle it correctly in Xen (see the vGICv3 re-dist example...).
> >
> > If we go on one vITS per multiple pITS we would have to send the command
> > SYNC/INVALL to every pITS.
> >
> > Regards,
> >
> > --
> > Julien Grall