Re: [Qemu-devel] [Xen-devel] [BUG 1747]Guest could't find bootable device with memory more than 3600M

[George Dunlap <george.dunlap@eu.citrix.com>]

On 14/06/13 15:14, George Dunlap wrote:
> On 14/06/13 12:34, Ian Campbell wrote:
>> On Fri, 2013-06-14 at 11:53 +0100, George Dunlap wrote:
>>> On Thu, Jun 13, 2013 at 6:22 PM, Ian Campbell 
>>> <Ian.Campbell@citrix.com> wrote:
>>>> On Thu, 2013-06-13 at 17:55 +0100, Stefano Stabellini wrote:
>>>>
>>>>>>> We could have a xenstore flag somewhere that enables the old 
>>>>>>> behaviour
>>>>>>> so that people can revert back to qemu-xen-traditional and make 
>>>>>>> the pci
>>>>>>> hole below 4G even bigger than 448MB, but I think that keeping 
>>>>>>> the old
>>>>>>> behaviour around is going to make the code more difficult to 
>>>>>>> maintain.
>>>>>> The downside of that is that things which worked with the old 
>>>>>> scheme may
>>>>>> not work with the new one though. Early in a release cycle when 
>>>>>> we have
>>>>>> time to discover what has broken then that might be OK, but is 
>>>>>> post rc4
>>>>>> really the time to be risking it?
>>>>> Yes, you are right: there are some scenarios that would have worked
>>>>> before that wouldn't work anymore with the new scheme.
>>>>> Are they important enough to have a workaround, pretty difficult to
>>>>> identify for a user?
>>>> That question would be reasonable early in the development cycle. 
>>>> At rc4
>>>> the question should be: do we think this problem is so critical 
>>>> that we
>>>> want to risk breaking something else which currently works for people.
>>>>
>>>> Remember that we are invalidating whatever passthrough testing people
>>>> have already done up to this point of the release.
>>>>
>>>> It is also worth noting that the things which this change ends up
>>>> breaking may for all we know be equally difficult for a user to 
>>>> identify
>>>> (they are after all approximately the same class of issue).
>>>>
>>>> The problem here is that the risk is difficult to evaluate, we just
>>>> don't know what will break with this change, and we don't know 
>>>> therefore
>>>> if the cure is worse than the disease. The conservative approach at 
>>>> this
>>>> point in the release would be to not change anything, or to change the
>>>> minimal possible number of things (which would preclude changes which
>>>> impact qemu-trad IMHO).
>>>>
>>>
>>>> WRT pretty difficult to identify -- the root of this thread 
>>>> suggests the
>>>> guest entered a reboot loop with "No bootable device", that sounds
>>>> eminently release notable to me. I also not that it was changing the
>>>> size of the PCI hole which caused the issue -- which does somewhat
>>>> underscore the risks involved in this sort of change.
>>> But that bug was a bug in the first attempt to fix the root problem.
>>> The root problem shows up as qemu crashing at some point because it
>>> tried to access invalid guest gpfn space; see
>>> http://lists.xen.org/archives/html/xen-devel/2013-03/msg00559.html.
>>>
>>> Stefano tried to fix it with the above patch, just changing the hole
>>> to start at 0xe; but that was incomplete, as it didn't match with
>>> hvmloader and seabios's view of the world.  That's what this bug
>>> report is about.  This thread is an attempt to find a better fix.
>>>
>>> So the root problem is that if we revert this patch, and someone
>>> passes through a pci device using qemu-xen (the default) and the MMIO
>>> hole is resized, at some point in the future qemu will randomly die.
>> Right, I see, thanks for explaining.
>>
>>> If it's a choice between users experiencing, "My VM randomly crashes"
>>> and experiencing, "I tried to pass through this device but the guest
>>> OS doesn't see it", I'd rather choose the latter.
>> All other things being equal, obviously we all would. But the point I've
>> been trying to make is that we don't know the other consequences of
>> making that fix -- e.g. on existing working configurations. So the
>> choice is "some VMs randomly crash, but other stuff works fine and we
>> have had a reasonable amount of user testing" and "those particular VMs
>> don't crash any more, but we don't know what other stuff no longer works
>> and the existing test base has been at least partially invalidated".
>>
>> I think that at post rc4 in a release we ought to be being pretty
>> conservative about the risks of this sort of change, especially wrt
>> invalidating testing and the unknowns involved.
>>
>> Aren't the configurations which might trip over this issue are going to
>> be in the minority compared to those which we risk breaking?
>
> So there are the technical proposals we've been discussing, each of 
> which has different risks.
>
> 1. Set the default MMIO hole size to 0xe0000000.
> 2. If possible, relocate PCI devices that don't fit in the hole to the 
> 64-bit hole.
>  - Here "if possible" will mean a) the device has a 64-bit BAR, and b) 
> this hasn't been disabled by libxl (probably via a xenstore key).
> 3. If possible, resize the MMIO hole; otherwise refuse to map the device
>  - Currently "if possible" is always true; the new thing here would be 
> making it possible for libxl to disable this, probably via a xenstore 
> key.
>
> Each of these will have different risks for qemu-traditional and 
> qemu-xen.
>
> Implementing #3 would have no risk for qemu-traditional, because we 
> won't be changing the way anything works; what works will still work, 
> what is broken (if anything) will still be broken.
>
> Implementing #3 for qemu-xen only changes one kind of failure for 
> another.  If you resize the MMIO hole for qemu-xen, then you *will* 
> eventually crash.  So this will not break existing working 
> configurations -- it will only change the failure from "qemu crashes 
> at some point" to "the guest OS cannot see the device". This is a 
> uniform improvement.

I suppose this is not strictly true.  If you resize the MMIO hole *such 
that it overlaps what was originally guest memory*, then it will crash.  
If you have a smaller guest with say, only 1 or 2GiB of RAM, then you 
can probably resize the MMIO hole arbitrarily on qemu-xen and have no 
ill effects.  So as stated ("never resize MMIO hole"), this would cause 
some successes into "guest can't see the device" failures.

(Stefano, correct me if I'm wrong here.)

But hvmloader should know whether this is the case, however, because if 
there is memory there it has to relocate it.  So we should change "is 
possible" to mean, "if we don't need to relocate memory, or if 
relocating memory has been enabled by libxl".

  -George