Re: [Xen-devel] [PATCH 1 of 2] Global virq for low memory situations

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Andres Lagar-Cavilla [mailto:andres@xxxxxxxxxxxxxxxx]
> Subject: RE: [Xen-devel] [PATCH 1 of 2] Global virq for low memory situations
> 
> >> From: Andres Lagar-Cavilla [mailto:andres@xxxxxxxxxxxxxxxx]
> >> Sent: Wednesday, February 29, 2012 7:26 PM
> >> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> >> Cc: ian.campbell@xxxxxxxxxx; andres@xxxxxxxxxxxxxx; tim@xxxxxxx;
> >> JBeulich@xxxxxxxx;
> >> ian.jackson@xxxxxxxxxx; adin@xxxxxxxxxxxxxx
> >> Subject: [Xen-devel] [PATCH 1 of 2] Global virq for low memory
> >> situations
> >>
> >>  xen/common/page_alloc.c  |  112
> >> +++++++++++++++++++++++++++++++++++++++++++++++
> >>  xen/include/public/xen.h |    1 +
> >>  2 files changed, 113 insertions(+), 0 deletions(-)
> >
> > Just a note that I think this will trigger false positives when
> > Xen tmem is working (with tmem-enabled guests).  With tmem,
> > Xen has two classes of free memory: free and "freeable".
> > In many situations, free will approach 0 but there will still
> > be lots of "freeable" memory which is available for guest
> > ballooning, launching new domains, etc.
> >
> > It may be the case that your virq may never be used on
> > a system with a tmem guest, but if it is, weird things
> > (like unnecessary hypervisor swapping) may happen.
> 
> Since I know zero point zero zero zip nada about tmem, maybe the most
> efficient path is that you throw on top of the patch the tmem magic. Which
> would seem to be all about factoring tmem_freeable_pages() into the
> calculation?

Since I know less than zero (:-) about what you plan to use
the virq for, and neither of us has tested one in the presence
of the other, the sane short term solution may be to at least
WARN_ON_ONCE and possibly disable your virq if opt_tmem is set.
For now, tmem requires an explicit boot option (which sets opt_tmem)
so you should be safe if opt_tmem==0.

Longer term, we are going to need to test environments where
both xenpaging/sharing is enabled for legacy/Windows HVM guests
AND tmem is enabled for tmem-savvy guests.

Does that make sense?

Dan



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