[PATCH] xen/dom0less: Clear xenstore page

[Jason Andryuk <jason.andryuk@xxxxxxx>]

C Xenstored was seen failing to map a domU's event channel.  Enhancing
Xen's error messages shows:
common/event_channel.c:427:d0v0 EVTCHNOP failure: d1, port 3267543746 error -22

Port 3267543746 is 0xc2c2c2c2 - SCRUB_PATTERN.

Xenstore expects a non-zero evtchn_port to be valid.  Explicitly zero
the page to avoid this.

Fixes: f78895ad78c9 ("tools/xenstored: Read event channel from xenstored page")
Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
---
Tweaked stubdom & dom0less C xenstored pipeline success:
Test CI pipeline:
https://gitlab.com/xen-project/people/jandryuk-amd/xen/-/pipelines/1994879466

Example failure:
https://gitlab.com/xen-project/people/jandryuk-amd/xen/-/jobs/11084356847

Alternatively, a fixes could be:
Fixes: df2c03e2ebfe ("xen/arm: Alloc XenStore page for Dom0less DomUs from 
hypervisor")
If the page should have been zero from the start.

gitlab-ci uses oxenstored for most testings, so this isn't seen by
default.  I don't see this in local testing, so I guess I don't have
SCRUB_PATTERN writing enabled?
---
 xen/common/device-tree/dom0less-build.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/common/device-tree/dom0less-build.c 
b/xen/common/device-tree/dom0less-build.c
index 6bb038111d..badc227031 100644
--- a/xen/common/device-tree/dom0less-build.c
+++ b/xen/common/device-tree/dom0less-build.c
@@ -600,6 +600,7 @@ static int __init alloc_xenstore_page(struct domain *d)
     d->arch.hvm.params[HVM_PARAM_STORE_PFN] = gfn_x(gfn);
 #endif
     interface = map_domain_page(mfn);
+    clear_page(interface);
     interface->connection = XENSTORE_RECONNECT;
     unmap_domain_page(interface);
 
-- 
2.50.1