Re: [Xen-devel] PVH dom0 creation fails - the system freezes

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jul 25, 2018 at 05:05:35PM +0300, bercarug@xxxxxxxxxx wrote:
> On 07/25/2018 05:02 PM, Wei Liu wrote:
> > On Wed, Jul 25, 2018 at 03:41:11PM +0200, Juergen Gross wrote:
> > > On 25/07/18 15:35, Roger Pau Monné wrote:
> > > > > What could be causing the available memory loss problem?
> > > > That seems to be Linux aggressively ballooning out memory, you go from
> > > > 7129M total memory to 246M. Are you creating a lot of domains?
> > > This might be related to the tools thinking dom0 is a PV domain.
> > Good point.
> > 
> > In that case, xenstore-ls -fp would also be useful. The output should
> > show the balloon target for Dom0.
> > 
> > You can also try to set the autoballoon to off in /etc/xen/xl.cfg to see
> > if it makes any difference.
> > 
> > Wei.
> Also tried setting autoballooning off, but it had no effect.

This is a Linux/libxl issue that I'm not sure what's the best way to
solve. Linux has the following 'workaround' in the balloon driver:

err = xenbus_scanf(XBT_NIL, "memory", "static-max", "%llu",
                   &static_max);
if (err != 1)
        static_max = new_target;
else
        static_max >>= PAGE_SHIFT - 10;
target_diff = xen_pv_domain() ? 0
                : static_max - balloon_stats.target_pages;

I suppose this is used to cope with the memory reporting mismatch
usually seen on HVM guests. This however interacts quite badly on a
PVH Dom0 that has for example:

/local/domain/0/memory/target = "8391840"   (n0)
/local/domain/0/memory/static-max = "17179869180"   (n0)

One way to solve this is to set target and static-max to the same
value initially, so that target_diff on Linux is 0. Another option
would be to force target_diff = 0 for Dom0.

I'm attaching a patch for libxl that should solve this, could you
please give it a try and report back?

I'm still unsure however about the best way to fix this, need to think
about it.

Roger.
---8<---
diff --git a/tools/libxl/libxl_mem.c b/tools/libxl/libxl_mem.c
index e551e09fed..2c984993d8 100644
--- a/tools/libxl/libxl_mem.c
+++ b/tools/libxl/libxl_mem.c
@@ -151,7 +151,9 @@ retry_transaction:
         *target_memkb = info.current_memkb;
     }
     if (staticmax == NULL) {
-        libxl__xs_printf(gc, t, max_path, "%"PRIu64, info.max_memkb);
+        libxl__xs_printf(gc, t, max_path, "%"PRIu64,
+                         libxl__domain_type(gc, 0) == LIBXL_DOMAIN_TYPE_PV ?
+                         info.max_memkb : info.current_memkb);
         *max_memkb = info.max_memkb;
     }
 


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