Re: [Xen-devel] Code freeze exception for Xen 4.9 ARM ITS emulation?

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 7 Apr 2017, Andre Przywara wrote:
> Hello!
> 
> The development of the ARM ITS emulation support has taken more time
> than anticipated and I won't be able to address and fix all outstanding
> comments until the official code freeze date.
> 
> However the first part of the series is in a good shape and should be
> ready to be merged in time.

Patches 1-9 are committed.


> I was wondering if I could be granted a brief code freeze exception for
> the remaining part of the series?
> I would address the outstanding issues until Thursday, April 13th and
> would post (a) new version(s) on the ML by then.
> Needless to say, that regardless of this, I will be promptly reacting
> and fixing and potential regressions introduced by the series.

The remaining patches are not bad, but there are still few holes. I'll
have to see the new versions with those holes plugged before I can make
a decision on whether the series is appropriate for 4.9. The only thing
I can say today is that it looks difficult but possible.

Of course, even if I think it's acceptable, for the code to make it into
Xen 4.9 past the code freeze, Julien will have to assess the risks, make
up his mind and, as release manager, he will have the final say.


> The idea would be to split the series and merge the first patches by the
> code freeze.
> I would then address the remaining comments and the second part of the
> series, consisting of patches to new files, could be merged by next
> week.
> I tuned the common code to make sure we only enter new code paths
> if the code is compiled in and the machine actually has the ITS
> hardware, to minimize the risk of breaking existing setups.
> 
> 
> Rationale for the exception:
> ------------------------------
> - This feature is a technical preview, disabled by default and
> requiring the EXPERT Kconfig option to be actually compiled in.
> - There is a strong interest from various parties to have the ITS
> emulation support in the Xen 4.9 release. Without the ITS a number of
> ARM servers will not be able to use MSIs (even in Dom0), which would
> prevent some systems from using PCI devices at all.
> - Xen's architecture requires the ITS emulation code to be usable for
> Dom0 already, so this is not about device passthrough to DomUs, but
> about to get PCI devices running. At the moment some servers can't use
> any network or block storage devices in Dom0, which would be fixed by
> this series.
> - The technical preview would allow various people to easily test this
> code, helping to gain confidence in this and to find potential bugs.
> 
> 
> Potential risks:
> ----------------
> - The code touches existing GIC emulation code, which is used by all ARM
> machines so far. However the ITS is an extension to the GICv3 interrupt
> controller, so the new code will be mostly adding functions, and changes
> existing code only in a very few places. Most (if not all) of those
> points can be secured by a gating variable (d->arch.vgic.has_its), which
> would never be set when the code is not configured in or the machine is
> not using the ITS.
> 
> Please let me know if you need further information or I can be of more
> assistance!
> 
> Thank you very much!
> 
> Cheers,
> Andre.
> 

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