[PATCH v2] tools/libxl: make default of max event channels dependant on vcpus

[Juergen Gross <jgross@suse.com>]

Since Xen 4.4 the maximum number of event channels for a guest is
defaulting to 1023. For large guests with lots of vcpus this is not
enough, as e.g. the Linux kernel uses 7 event channels per vcpu,
limiting the guest to about 140 vcpus.

Instead of requiring to specify the allowed number of event channels
via the "event_channels" domain config option, make the default
depend on the maximum number of vcpus of the guest. This will require
to use the "event_channels" domain config option in fewer cases as
before.

As different guests will have differing needs the calculation of the
maximum number of event channels can be a rough estimate only,
currently based on the Linux kernel requirements. Nevertheless it is
much better than the static upper limit of today as more guests will
boot just fine with the new approach.

In order not to regress current configs use 1023 as the minimum
default setting.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
V2:
- use max() instead of min()
- clarify commit message a little bit
---
 tools/libxl/libxl_create.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/libxl/libxl_create.c b/tools/libxl/libxl_create.c
index e7cb2dbc2b..c025b21894 100644
--- a/tools/libxl/libxl_create.c
+++ b/tools/libxl/libxl_create.c
@@ -226,7 +226,7 @@ int libxl__domain_build_info_setdefault(libxl__gc *gc,
             b_info->iomem[i].gfn = b_info->iomem[i].start;
 
     if (!b_info->event_channels)
-        b_info->event_channels = 1023;
+        b_info->event_channels = max(1023, b_info->max_vcpus * 8 + 255);
 
     libxl__arch_domain_build_info_setdefault(gc, b_info);
     libxl_defbool_setdefault(&b_info->dm_restrict, false);
-- 
2.16.4