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Re: [PATCH v2] xen/arm: implement GICD_I[S/C]ACTIVER reads

On Wed, Apr 1, 2020 at 10:54 AM Bertrand Marquis <Bertrand.Marquis@xxxxxxx> wrote:

On 1 Apr 2020, at 09:30, Julien Grall <julien@xxxxxxx> wrote:

On 01/04/2020 01:57, Stefano Stabellini wrote:
On Mon, 30 Mar 2020, Julien Grall wrote:
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
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.
Writing on the mailing list some of our discussions today.
Correctness is not just in terms of compliance to a specification but it
is also about not breaking guests. Introducing latency in the range of
milliseconds, or hundreds of microseconds, would break any latency
sensitive workloads. We don't have numbers so we don't know for certain
the effect that your suggestion would have.

You missed part of the discussion. I don't disagree that latency is important. However, if an implementation is only 95% reliable, then it means 5% of the time your guest may break (corruption, crash, deadlock...). At which point the latency is the last of your concern.

It would be interesting to have those numbers, and I'll add to my TODO
list to run the experiments you suggested, but I'll put it on the
back-burner (from a Xilinx perspective it is low priority as no
customers are affected.)

How about we get a correct implementation merge first and then discuss about optimization? This would allow the community to check whether there are actually noticeable latency in their workload.


I am not sure that pushing something with a performance impact to later fix it is the right approach here.

The patch is an improvement compared to the current code and it can be further improved later to handle more cases (other cores).

If we really have to sync all vCPUs here, this will cost a lot and the result will still be the status in the past in fact because nothing will make sure that at the point the guest gets back the value it is still valid.

The same is true on real hardware, right?

Looking at this discussion as a non-ARM person, it sounds a bit like ARM specified this in a way that was useless-but-easy-to-implement-so-why-not; but it turns out to be useless-but-hard-to-implement virtualized.

On the one hand, I'm sympathetic to Julien's point of view; I very much don't like the idea of silently changing behavior just because the specified behavior is inconvenient for us.

On the other hand, I'm sympathetic to Stefano's point of view, that it's pointless to introduce a load of overhead and jitter to implement behavior which nobody in their right mind would even want to use (at least virtualized).

What I think would be *ideal* would be for ARM to change the specification to make it easier virtualize.  For instance:, by specifying that the register *may* contain information about other cores, but may not; or, that the register will contain information on other cores on real hardware, but not virtualized.

Barring that, I think we should have a centralized place to document deviations from the spec; and I think changes to this spec should be coordinated with KVM (and maybe ACRN?), to minimize hypervisor-specific deviations.





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