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

[Jürgen Groß <jgross@xxxxxxxx>]

On 28.11.19 09:48, Julien Grall wrote:
> Hi,
>
> On 28/11/2019 08:32, Jürgen Groß wrote:
> > On 28.11.19 09:14, Julien Grall wrote:
> > > > 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.
> > Sorry, but this is a rather weird statement.
> >
> > IIUC you are saying that a typo blocking boot of Linux 5.4 should not be
> > fixed as you are not sure there are no other bugs?
> The implementation of I*ACTIVER was incorrect but gone unnoticed because 
> no-one used it until 5.4. It also happen that we didn't cover all the 
> I*ACTIVER registers, so 5.4 crashes instead of using the wrong behavior.
> This patch is basically replacing a guest crash by a behavior we don't 
> fully understand.
> > > 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.
> > Or do you think that with the typo fixed and running Linux 5.4 guests
> > will destabilize the host?
> It is not going to destabilize the hosts. But this is not going to make 
> 5.4 running correctly as Xen guest.
Have you verified it isn't running correctly or do you just think it
could hit problems?

In both cases I see no reason to keep wrong code.

Either the patch will let run Linux 5.4 fine - then the patch should
definitely be taken.

Or the patch will let Linux 5.4 boot further, but some problems will
occur. Then it will be possible to analyze those problems and try to
fix them, very possibly with the sane approach you are hoping for.


Juergen

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