Re: [PATCH] s390: PCI: fix IOMMU region init

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 9/26/19 10:34 AM, Peter Maydell wrote:
> On Thu, 26 Sep 2019 at 15:12, Matthew Rosato <mjrosato@linux.ibm.com> wrote:
>>
>> The fix in dbe9cf606c shrinks the IOMMU memory region to a size
>> that seems reasonable on the surface, however is actually too
>> small as it is based against a 0-mapped address space.  This
>> causes breakage with small guests as they can overrun the IOMMU window.
>>
>> Let's go back to the prior method of initializing iommu for now.
>>
>> Fixes: dbe9cf606c ("s390x/pci: Set the iommu region size mpcifc request")
>> Reported-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
>> Reported-by: Stefan Zimmerman <stzi@linux.ibm.com>
>> Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
> 
> So in commit f0a399dbae6a2d0e2 (Nov 2015) we used "pal - pba + 1".
> In commit f7c40aa1e7feb50bc4 (June 2016) we switched to "pal + 1".
> In commit dbe9cf606c (Jan 2019) we went back to "pal - pba + 1"
> Now we're on "pal + 1" again...
> 
> Are we really sure that this is correct and that we're not
> just going to keep looping around between these two formations
> forever? :-)
> 

Yes :) -- Pierre's RB comment sums it up pretty well, until we change
the way the address space is mapped it is not safe to use pal - pba + 1.
 This was noted in f7c40aa1e and then erroneously missed in dbe9cf606c.
 With this, small guests break immediately (PCI base is higher than the
IOMMU region can handle).  Larger guests don't break immediately but can
break later if their PCI space usage pushes high enough (their IOMMU
region can handle pba, but somewhere < pal).

The comment block added was to help assist in keeping further hands off
of this call until such a time where the address space mapping is changed.

> thanks
> -- PMM
>