[libvirt] [PATCH] qemuBuildMemoryBackendStr: Handle one more corner case

[Michal Privoznik]

On 08/10/2017 03:44 PM, Martin Kletzander wrote:
> On Tue, Aug 08, 2017 at 05:04:15PM +0200, Michal Privoznik wrote:
>> https://bugzilla.redhat.com/show_bug.cgi?id=1458638
>>
>> This code is so complicated because we allow enabling the same
>> bits at many places. Just like in this case: huge pages can be
>> enabled by global <hugepages/> element under <memoryBacking> or
>> on per <memory/> basis. To complicate things a bit more, users
>> are allowed to not specify any specific page size in which case
> 
> s/to not specify/not to specify/
> 
> reads a bit nicely, or even
> 
> s/to not specify any/to specify no/ or "allowed to omit the page size"

I'm going with the latter.

> 
> Choice is yours
> 
> Reviewed-by: Martin Kletzander <mkletzan at redhat.com>

Pushed, thanks.

> 
> One more note that I told you in person, but I can't resist the urge to
> share it more widely.  The code is gross because it can't be made much
> better due to the fact that we keep the parsed data the same way they
> are in the XML.  I think this could be made nicer if we were to fill all
> the information during parsing.
> 
> When parsing /memory/hugepages we would fill in missing information
> right away.  Then, when parsing /cpu/numa/cell we could copy that
> information to the specific cells (if missing) and because the whole
> definition is already available from parsing /memory/hugepages there's
> less lookups.  Finally, when parsing /devices/memory/source we can
> directly get missing info from the numa cell if nodemask is present or
> global hugepages if it is not, and we don't have to lookup anything,
> because it is already guaranteed to be filled in for the previous parts.
> 
> I hope it's understandable.  What I hope even more is that someone
> refactors the code this way =)

Right. One thing though that I haven't realized during our conversation
is that we would need to keep track if the info is supplied by user or
by this algorithm. The problem is the following: users can specify
<memoryBacking> <hugepages/> </memoryBacking>  in which case so called
default huge page size is used. The default is whatever page size is
printed in /proc/meminfo. For instance, on my local machine:

$ cat /proc/meminfo | grep Hugepagesize:
Hugepagesize:       2048 kB

$ cat /proc/cpuinfo | grep -o pdpe1gb
pdpe1gb

So the default huge page size is 2M but the processor supports also 1GB.
However, on different machine the default may be something else. 2MB
might not even be accessible there. Therefore if we would fill that info
during parsing we might be screwed. Alternatively, if admin have 2MB
enabled during define, but then prohibits 2MB in favour of 1GB, we are
back in the problem. Hope it makes sense what I'm saying.

Michal