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

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(a)suse.com&gt; --- 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;