[PATCH 1/1] qapi/dump: add allowed-by-guest feature to win-dmp

Commit 1c0e259c5a35 ("dump: make win_dump_available() check vmcoreinfo
for a Windows dump header") changed two things in a way that is visible
to QMP clients but not to introspection:
query-dump-guest-memory-capability now lists win-dmp only for a guest
that has published a Windows dump header through the vmcoreinfo device,
and dump-guest-memory, which shares win_dump_available(), rejects the
format otherwise. Before that, both accepted win-dmp on any x86
machine.

A client that wants to select win-dmp automatically therefore cannot
trust the capability query on its own: on an older QEMU it reports
win-dmp for every x86 guest, Linux ones included, where the resulting
dump is useless. libvirt ran into exactly this while picking a format
for on_crash and watchdog triggered dumps, and has no way to tell the
two behaviours apart.

Add an 'allowed-by-guest' feature to the win-dmp member of
DumpGuestMemoryFormat so the fixed behaviour becomes discoverable.
DumpGuestMemoryFormat is reachable from both
query-dump-guest-memory-capability's return type and
dump-guest-memory's arguments, so a single flag covers both halves of
the change. Where the feature is absent, a reported win-dmp says
nothing about the guest, and a client that needs the dump to be
loadable afterwards should fall back to elf.

CC: Eric Blake <[email protected]>
CC: Markus Armbruster <[email protected]>
CC: "Marc-André Lureau" <[email protected]>
Suggested-by: Daniel P. Berrangé <[email protected]>
Signed-off-by: Denis V. Lunev <[email protected]>
---
libvirt discussion:
https://lists.libvirt.org/archives/list/[email protected]/thread/7QBRM2GOC7M5E7EAIE44JNWBSKNN62BC/

 qapi/dump.json | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/qapi/dump.json b/qapi/dump.json
index 726b520870..690f3963fe 100644
--- a/qapi/dump.json
+++ b/qapi/dump.json
@@ -38,6 +38,13 @@
 # @win-dmp: Windows full crashdump format, can be used instead of ELF
 #     converting (since 2.13)
 #
+# Features:
+#
+# @allowed-by-guest: If present, @win-dmp is listed by
+#     `query-dump-guest-memory-capability`, and accepted by
+#     `dump-guest-memory`, only when the guest has published a Windows
+#     dump header through the vmcoreinfo device (since 11.1)
+#
 # Since: 2.0
 ##
 { 'enum': 'DumpGuestMemoryFormat',
@@ -45,7 +52,7 @@
       'elf',
       'kdump-zlib', 'kdump-lzo', 'kdump-snappy',
       'kdump-raw-zlib', 'kdump-raw-lzo', 'kdump-raw-snappy',
-      'win-dmp' ] }
+      { 'name': 'win-dmp', 'features': ['allowed-by-guest'] } ] }
 
 ##
 # @dump-guest-memory:
-- 
2.53.0