Re: Xen/arm: Virtual ITS command queue handling

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

On Fri, 2015-05-15 at 18:44 +0530, Vijay Kilari wrote:
> On Fri, May 15, 2015 at 6:23 PM, Ian Campbell <ian.campbell@citrix.com> wrote:
> > 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.
> 
> Not only at the end of batch, SYNC can be appended based on every
> command within the batch.

Could be, but something to avoid I think?

> Also to handle second bullet, where a batch of commands might be
> sent on multple pITS. In that case batch of ITS commands is split
> across pITS and we have
> to wait for all the pITS to complete. Managing this would be difficult.
> For this I propose, batch can be created/split such that each batch
> contains commands related to one pITS. But it leads to small batch of commands.

That's not a bad idea, commonly I would expect commands for one device
to come in a short batch anyway. So long as the thing does cope if not I
think this might work.

> 
> >> > 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
> >
> >
> 
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