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Re: [libvirt] [Xen-devel] [PATCH] libxl: fix dom0 autoballooning with Xen 4.8

On 02/02/2017 04:42 AM, Wei Liu wrote:
I saw this mail but didn't realise it required my input, sorry.

I suppose it didn't and I was shamelessly fishing for a review - so you have my apologies :-). But the patch does fix an annoying, easily encountered bug which I'm eager to see resolved in the next libvirt release.

On Wed, Feb 01, 2017 at 04:00:54PM -0700, Jim Fehlig wrote:
Jim Fehlig wrote:
xen.git commit 57f8b13c changed several of the libxl memory
get/set functions to take 64 bit parameters. The libvirt
libxl driver still uses uint32_t variables for these various
parameters, which is particularly problematic for the
libxl_set_memory_target() function.

When dom0 autoballooning is enabled, libvirt (like xl) determines
the memory needed to start a domain and the memory available. If
memory available is less than memory needed, dom0 is ballooned
down by passing a negative value to libxl_set_memory_target()
'target_memkb' parameter. Prior to xen.git commit 57f8b13c,
'target_memkb' was an int32_t. Subtracting a larger uint32 from
a smaller uint32 and assigning it to int32 resulted in a negative
number. After commit 57f8b13c, the same subtraction is widened
to a int64, resulting in a large positive number. The simple
fix taken by this patch is to assign the difference of the
uint32 values to a temporary int32 variable, which is then
passed to 'target_memkb' parameter of libxl_set_memory_target().

Note that it is undesirable to change libvirt to use 64 bit
variables since it requires setting LIBXL_API_VERSION to 0x040800.
Currently libvirt supports LIBXL_API_VERSION >= 0x040400,
essentially Xen >= 4.4.

Any comments on this patch? xen-devel is already in cc, but I'll add Wei (one of
the Xen tools maintainers) and Juergen (author of commit 57f8b13c) for some
additional exposure :-).

Currently, "out-of-the-box" Xen >= 4.8 + libvirt setup (dom0 autoballooning
enabled) is broken without this fix.


Signed-off-by: Jim Fehlig <jfehlig suse com>
 src/libxl/libxl_domain.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/libxl/libxl_domain.c b/src/libxl/libxl_domain.c
index a5314b0..ed73cd2 100644
--- a/src/libxl/libxl_domain.c
+++ b/src/libxl/libxl_domain.c
@@ -909,6 +909,7 @@ libxlDomainFreeMem(libxl_ctx *ctx, libxl_domain_config *d_config)
     uint32_t needed_mem;
     uint32_t free_mem;
+    int32_t target_mem;
     int tries = 3;
     int wait_secs = 10;

@@ -922,7 +923,8 @@ libxlDomainFreeMem(libxl_ctx *ctx, libxl_domain_config *d_config)
         if (free_mem >= needed_mem)
             return 0;

-        if (libxl_set_memory_target(ctx, 0, free_mem - needed_mem,
+        target_mem = free_mem - needed_mem;
+        if (libxl_set_memory_target(ctx, 0, target_mem,

This should do the trick.

Thanks. There are other ways to skin this cat and if folks prefer one of those I can push a followup. But for now, so that we have a fix, I'll push this variant.


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