Re: [Xen-devel] [RFC 01/16] xen/arm: Re-enable interrupt later in the trap path

[Julien Grall <julien.grall@xxxxxxx>]

(+ Andre)

On 7/30/19 6:34 PM, Andrii Anisov wrote:
> Hello Julien,
>
> It looks I missed answering this email. So do this now:
>
> On 29.11.18 00:00, Julien Grall wrote:
> > On 28/11/2018 21:31, Andrii Anisov wrote:
> > > From: Julien Grall <julien.grall@xxxxxxx>
> > >
> > > This makes function enter_hypervisor_head() being executed with
> > > irqs locked. This also give a fine side effect - it assures that
> > > LRs are cleared prior to any IRQs processing, which leads to a
> > > better (faster) IRQs processing.
> > Again, this need some rationale why is it faster. And how this is going
> > to impact the other vGIC...
> With the current code, there is a chance that hypervisor performs 
> do_IRQ() before fetching LRs with the information about IRQs being 
> processed by a guest. This could happen in guest trap paths where 
> interrupts are enabled before `enter_hypervisor_head()` call.
> Performing `do_IRQ()` prior to `vgic_sync_from_lrs()` is assumed as less 
> efficient than doing it vice versa for high IRQ rate use-cases with the 
> number of different interrupts (specific for multimedia scenarios).
What you suggest here is really specific to your use case. However, 
think about the Real-Time case where you may want to receive your 
interrupt as soon as possible.
In general, keeping the interrupt disabled for a long time is a pretty 
bad idea because you increase latency. Interrupts should only be 
disabled when it is strictly necessary.
At the moment I don't view this change as necessary. But I am happy to 
be proven otherwise.
>   - For the old vgic implementation, `vgic_sync_from_lrs()` release LRs 
> from processed interrupts also shortens `inflight_irqs` sorted list. So 
> `do_IRQ()` has better chances to write IRQ directly to LR instead of 
> saving it into `lr_pending` list and possibly faster insert the new IRQ 
> into `inflight_irqs` list.
Saving to lr_pending means that you have all the LRs full.

I vaguely recall we spoke about it before but I can't find anything in 
my inbox. So can you explain again your use-case, the number of LRs...?
Note this is were writing down everything in the commit message (or 
after ---) helps people to understand where you are coming from.
Cheers,


--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel