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

[Jim Fehlig]

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.

Signed-off-by: Jim Fehlig <jfehlig at 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,
                                     /* relative */ 1, 0) < 0)
             goto error;
 
-- 
2.9.2