[Pkg-xen-devel] Bug#880554: xen domu freezes with kernel linux-image-4.9.0-4-amd64

[Valentin Vidic]

On Sun, Jan 07, 2018 at 07:36:40PM +0100, Hans van Kranenburg wrote:
> Recently a tool was added to "dump guest grant table info". You could
> see if it compiles on the 4.8 source and see if it works? Would be
> interesting to get some idea about how high or low these numbers are in
> different scenarios. I mean, I'm using 128, you 256, and we even don't
> know if the actual value is maybe just above 32? :]
> 
> https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=df36d82e3fc91bee2ff1681fd438c815fa324b6a

The diag tool does not build inside xen-4.8:

xen-diag.c: In function ‘gnttab_query_size_func’:
xen-diag.c:50:10: error: implicit declaration of function ‘xc_gnttab_query_size’ [-Werror=implicit-function-declaration]
     rc = xc_gnttab_query_size(xch, &query);
          ^~~~~~~~~~~~~~~~~~~~

but I think the same info is available in the thread on xen-devel:

  https://www.mail-archive.com/xen-devel@lists.xen.org/msg116910.html

When the domU hangs crash reports nr_grant_frames=32. After increasing
the gnttab_max_frames=256 the domU reports using nr_grant_frames=59.

So the new default of gnttab_max_frames=64 might be a bit close to 59,
but I suppose 128 would be just as safe as 256 I currently use (if
you prefer 128).

> If this is something users are going to run into while not doing more
> unusual things like having dozens of vcpus or network interfaces, then
> changing the default could prevent hours of frustration and debugging
> for them.

Yes, the failure case is quite nasty, as the domU just hangs without
even suggesting grant frames might be the problem. Not sure if domU
can detect this situation at all?

Anyway, if the value cannot be increased, the situation should at least
be mentioned in the NEWS.Debian of the xen package.

-- 
Valentin