Re: [Xen-devel] [ARM] Native application design and discussion (I hope)

[Julien Grall <julien.grall@xxxxxxx>]

On 05/09/2017 12:08 PM, Dario Faggioli wrote:
> On Tue, 2017-05-09 at 11:32 +0100, Julien Grall wrote:
> > Hi Dario,
> Hey,
>
> > On 05/09/2017 11:13 AM, Dario Faggioli wrote:
> > > Which, FTR, is what we do in Xen for a bunch of things already,
> > > i.e.,
> > > softirqs and tasklets.
> > No, we don't switch to the idle vCPU to handle tasklets or softirqs.
> > They will be done before entering to the guest and still in the
> > hypervisor context.
> Mmm... I don't know who's "we" here, but even if it's "you ARM people",
> you actually do.
>
> In fact, this is common code:
>
> static struct task_slice
> csched_schedule(
>     const struct scheduler *ops, s_time_t now, bool_t tasklet_work_scheduled)
> {
>     [...]
>     /* Choices, choices:
>      * - If we have a tasklet, we need to run the idle vcpu no matter what.
>     [...]
>     /* Tasklet work (which runs in idle VCPU context) overrides all else. */
>     if ( tasklet_work_scheduled )
>     {
>         TRACE_0D(TRC_CSCHED_SCHED_TASKLET);
>         snext = CSCHED_VCPU(idle_vcpu[cpu]);
>         snext->pri = CSCHED_PRI_TS_BOOST;
>     }
>     [...]
> }
>
> And this is "your" idle loop:
>
> void idle_loop(void)
> {
>     for ( ; ; )
>     {
>         [...]
>         local_irq_disable();
>         if ( cpu_is_haltable(smp_processor_id()) )
>         {
>             dsb(sy);
>             wfi();
>         }
>         local_irq_enable();
>         do_tasklet();
>         do_softirq();
>         [...]
>     }
> }
> Actually, yes, it was a bit inaccurate of me to cite both softirqs and
> tasklets, together, like I did. Softirqs indeed are checked and handled
> before leaving Xen, as you say, as well as, in the idle loop, as shown
> above.
>
> But for tasklet (and, to be 100% precise, for vCPU context tasklet),
> it's actually the case that we force the idle vCPU in execution to run
> them.
I am a bit confused. When I read the softirq code, I saw there is a 
softirq tasklet which call do_tasklet_work.
But it sounds like that tasklet can be either scheduled in softirq or in 
idle vCPU depending of the type of the tasklet (is_softirq).
Thank you for the explanation :).

> > > And it should not be hard to give such code access to the context
> > > of
> > > the vCPU that was previously running (in x86, given we implement
> > > what
> > > we call lazy context switch, it's most likely still loaded in the
> > > pCPU!).
> > I agree with Stefano, switching to the idle vCPU is a pretty bad
> > idea.
> >
> > the idle vCPU is a fake vCPU on ARM to stick with the common code
> > (we
> > never leave the hypervisor). In the case of the EL0 app, we want to
> > change exception level to run the code with lower privilege.
> >
> > Also IHMO, it should only be used when there are nothing to run and
> > not
> > re-purposed for running EL0 app.
> It's already purposed for running when there is nothing to do _or_ when
> there are tasklets.
>
> I do see your point about privilege level, though. And I agree with
> George that it looks very similar to when, in the x86 world, we tried
> to put the infra together for switching to Ring3 to run some pieces of
> Xen code.
We would like to do exactly the same for ARM. On another part of the 
thread I suggested to Volodymyr to look at what has been done there.
Cheers,

--
Julien Grall

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