Re: [Xen-devel] [PATCH V2] arch: arm: vgic-v3: fix GICD_ISACTIVER range

[Peng Fan <peng.fan@xxxxxxx>]

> Subject: Re: [Xen-devel] [PATCH V2] arch: arm: vgic-v3: fix GICD_ISACTIVER
> range
> 
> Hi Stefano,
> 
> On 28/11/2019 01:12, Stefano Stabellini wrote:
> > On Wed, 27 Nov 2019, Jürgen Groß wrote:
> >> On 27.11.19 10:31, Peng Fan wrote:
> >>>> Subject: Re: [Xen-devel] [PATCH V2] arch: arm: vgic-v3: fix
> >>>> GICD_ISACTIVER range
> >>>>
> >>>> On 27.11.19 01:01, Julien Grall wrote:
> >>>>> Hi,
> >>>>>
> >>>>> On 26/11/2019 23:17, Stefano Stabellini wrote:
> >>>>>> On Tue, 26 Nov 2019, Julien Grall wrote:
> >>>>>>> Hi,
> >>>>>>>
> >>>>>>> On 26/11/2019 20:43, Stefano Stabellini wrote:
> >>>>>>>> + Juergen
> >>>>>>>>
> >>>>>>>> I missed that you weren't in CC to the original patch, sorry.
> >>>>>>>> I think this patch should go in, as otherwise Linux 5.4 could
> >>>>>>>> run into problems. It is also a pretty straightforward 4 lines patch.
> >>>>>>>
> >>>>>>> 5.5 (or 5.6) is not going to run on Xen for other reasons (still
> >>>>>>> in the vGIC)... So I would not view this as critical.
> >>>>>>
> >>>>>> 5.5 is not out yet, in fact, the dev window has just opened.
> >>>>>> Isn't your statement a bit premature?
> >>>>>
> >>>>> The GICv4.1 work [1] is going to prevent Linux booting on all
> >>>>> current versions of Xen. While I can't confirm this is going to be
> >>>>> merged in 5.5, I can tell you this will break.
> >>>>>
> >>>>>>
> >>>>>> In any case, even if potential future Linux releases could have
> >>>>>> other additional issues, I don't think it should change our
> >>>>>> current view on this specific issue which affects 5.4, just released.
> >>>>>
> >>>>> The patch is definitely not as straightforward as you may think.
> >>>>> Please refer to the discussion we had on the first version. I
> >>>>> voiced concern about this approach and gave point what could go
> >>>>> wrong with
> >>>> happen.
> >>>>>
> >>>>> This patch may be better than the current state (i.e crashing),
> >>>>> but this wasn't tested enough to confirm this is the correct
> >>>>> things to do and no other bug will appear (I don't believe reading
> >>>>> I*ACTIVER was ever tested before).
> >>>>>
> >>>>> It is an annoying bug, but this is only affecting 5.4 which has
> >>>>> just been released. It feels to me this is a fairly risky choice
> >>>>> to merge it qutie late in the release without a good graps of the
> >>>>> problem (see above).
> >>>>>
> >>>>> So I would definitly, prefer if this patch is getting through
> >>>>> backport once we get more testing.
> >>>>>
> >>>>> We can still document the bug in the release note and point people
> >>>>> to the patch.
> >>>>>
> >>>>> Anyway, this is Juergen choice here. But at least now he has the
> >>>>> full picture...
> >>>>>
> >>>>> Cheers,
> >>>>>
> >>>>> [1]
> >>>>>
> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flwn.
> >>>>>
> >>>>
> net%2FArticles%2F800494%2F&data=02%7C01%7Cpeng.fan%40nxp.co
> >>>> m%7Cdca
> >>>>>
> >>>>
> dfb39240749ee675e08d772fcd3ba%7C686ea1d3bc2b4c6fa92cd99c5c30163
> >>>> 5%7C0%7
> >>>>>
> >>>>
> C0%7C637104302519996592&sdata=7Jv2IhI8HZgBTSuYzkEplFyhX1lzmv
> >>>> d73xb5
> >>>>> 2d6ERVQ%3D&reserved=0
> >>>>>
> >>>>
> >>>> Thanks, Julien, for sharing your opinion.
> >>>>
> >>>> With that statement I'd like to defer this patch to 4.14.
> >>>
> >>> But without this patch, 5.4 kernel will crash. So you prefer we
> >>> develop the solution as Julien suggested for 4.13?
> >>
> >> I certainly won't take a patch for 4.13 when a maintainer of the
> >> related code has reservations against it.
> >>
> >> I think the best thing to do is to develop a proper patch the
> >> maintainers are happy with and don't try to force it into 4.13 now.
> >> Such a patch can still be backported to 4.13 later.
> >
> > I chatted with Juergen and he explained to me something I didn't know
> > before. The release manager can only *block* a patch from being
> > committed, he/she cannot actually decide if the patch should be
> > committed or not for a given release. He/she cannot overrule a
> > maintainer either.
> >
> > In this case, Juergen cannot make the decision on whether the patch
> > should go in 4.13 or not.
> >
> > Although I couldn't reproduce the problem on Xilinx boards, I have to
> > take the community angle on this, and I would like to make sure our
> > releases work properly on any hardware, including NXP. Thus, I'll make
> > the case one more time, hoping that Julien might change his mind :-)
> 
> We had promise that patches to support NXP will be upstreamed, but this was
> never done. If you look at [1], there are a lot of patches on top of it.

That will happen after we rebase to 4.13. To support i.MX8 partition feature,
we introduced scfw api in XEN, this code might not be accepted by XEN
community, we have not find a good way for that.

Thanks,
Peng.

> 
> So I don't think NXP boot out-of-box and therefore I don't think we should
> make the decision based on this.

I not think this is only NXP issue although it only met by me currently.
It should be defer probe, but I have not look into the Linux code path.

Thanks,
Peng.

> 
> >
> > We know that the bug fix won't introduce any regressions because, as
> > Julien wrote, this code path was never used before. Also because of
> > that, waiting for the backport and more OSSTest runs won't make much
> > of a difference because OSSTest won't exercise this code path.
> 
> Conversely, we don't know how many regression this is going to be
> introducing for Linux 5.4 because this was only reproduced once and we know
> the implementation is incorrect.
> 
> But OSSTest is also testing different version of Linux and  5.4 should be
> tested soon (if not already), so we should also be able to exercise this code
> path.
> 
> >
> > It is true that the original code handling GICD_ISACTIVER was never
> > spec compliant, and it should be fixed properly. However, that is not
> > what this patch addresses. That code, in addition from not being spec
> > compliant by design, it also happens to have a typo. Fixing the typo
> > at this stage of the release is appropriate at least to get a
> > consistent behavior in the handling of GICD_ISACTIVER*, and also for
> > Linux 5.4 as guest. Not to give a false impression to users, the
> > warning ensures that the underlying Xen behavior is flagged appropriately.
> 
> Again, you have no promise that this will make the right thing for Linux 5.4.
> 
> This has been my point for the past week and ignored on the ground that
> some of the I*ACTIVER registers were implemented like that so we must
> continue to spread the false.
> 
> >
> > In short, I think the patch should go in now and there are no
> > downsides to it. That's it, I rest my case. Julien, I hope you'll 
> > reconsider.
> I don't really see the point to try to allow Linux 5.4 booting on Xen
> 4.13 without knowing whether we are not going to uncovered more BUG
> around I*ACTIVER.
> 
> If you really want this patch in Xen 4.13, then you should read my mail on the
> first version and trying to answer me why this 5.4 is going to be safe running
> on Xen 4.13.
> 
> If you can't answer that, then there are no reason to get this patch Xen 4.13.
> 
> I would have been happy to get this patch in next (not Xen 4.13), but this
> thread doesn't give me the confidence that someone sat done and fully
> understood the problem. So until someone is able to explain me whether this
> patch is safe:
> 
> NAcked-by: Julien Grall <julien@xxxxxxx>
> 
> Cheers,
> 
> --
> Julien Grall
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