Hi Stefano,
On 30/03/2020 17:35, Stefano Stabellini wrote:
On Sat, 28 Mar 2020, Julien Grall wrote:
qHi Stefano,
On 27/03/2020 02:34, Stefano Stabellini wrote:
This is a simple implementation of GICD_ICACTIVER / GICD_ISACTIVER
reads. It doesn't take into account the latest state of interrupts on
other vCPUs. Only the current vCPU is up-to-date. A full solution is
not possible because it would require synchronization among all vCPUs,
which would be very expensive in terms or latency.
Your sentence suggests you have number showing that correctly emulating the
registers would be too slow. Mind sharing them?
No, I don't have any numbers. Would you prefer a different wording or a
better explanation? I also realized there is a typo in there (or/of).
Let me start with I think correctness is more important than speed.
So I would have expected your commit message to contain some fact why
synchronization is going to be slow and why this is a problem.
To give you a concrete example, the implementation of set/way
instructions are really slow (it could take a few seconds depending on
the setup). However, this was fine because not implementing them
correctly would have a greater impact on the guest (corruption) and they
are not used often.
I don't think the performance in our case will be in same order
magnitude. It is most likely to be in the range of milliseconds (if not
less) which I think is acceptable for emulation (particularly for the
vGIC) and the current uses.
So lets take a step back and look how we could implement
ISACTIVER/ICACTIVER correctly.
The only thing we need is a snapshot of the interrupts state a given
point. I originally thought it would be necessary to use domain_pause()
which is quite heavy, but I think we only need the vCPU to enter in Xen
and sync the states of the LRs.
Roughly the code would look like (This is not optimized):
for_each_vcpu(d, v)
{
if ( v->is_running )
smp_call_function(do_nothing(), v->cpu);
}
/* Read the state */
A few remarks:
* The function do_nothing() is basically a NOP.
* I am suggesting to use smp_call_function() rather
smp_send_event_check_cpu() is because we need to know when the vCPU has
synchronized the LRs. As the function do_nothing() will be call
afterwards, then we know the the snapshot of the LRs has been done
* It would be possible to everything in one vCPU.
* We can possibly optimize it for the SGIs/PPIs case
This is still not perfect, but I don't think the performance would be
that bad.
Cheers,
--
Julien Grall
