kexec: don't use kexec_file_load on XEN

Since commit 29fe506 ("kexec: make -a the default")
kexec tries the kexec_file_load syscall first and only falls back to kexec_load on
selected error codes.

This effectively breaks kexec on XEN, unless -c is pecified to force the kexec_load
syscall.

The XEN-specific functions (xen_kexec_load / xen_kexec_unload) are only called
from my_load / k_unload, i.e. the kexec_load code path.

With -p (panic kernel) kexec_file_load on XEN fails with -EADDRNOTAVAIL (crash
kernel reservation is ignored by the kernel on XEN), which is not in the list
of return codes that cause the fallback to kexec_file.

Without -p kexec_file_load actualy leads to a kernel oops on v6.4.0
(needs to be dubugged separately).

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Fixes: 29fe506 ("kexec: make -a the default")
Signed-off-by: Simon Horman <horms@kernel.org>

kexec/kexec.8

@@ -162,6 +162,7 @@ Specify that the new kernel is of this
 .TP
 .BI \-s\ (\-\-kexec-file-syscall)
 Specify that the new KEXEC_FILE_LOAD syscall should be used exclusively.
+Ignored on XEN.
 .TP
 .BI \-c\ (\-\-kexec-syscall)
 Specify that the old KEXEC_LOAD syscall should be used exclusively.

kexec/kexec.c

@@ -1685,6 +1685,10 @@ int main(int argc, char *argv[])
 			}
 		}
 	}
+	if (xen_present()) {
+		do_kexec_file_syscall = 0;
+		do_kexec_fallback = 0;
+	}
 	if (do_kexec_file_syscall) {
 		if (do_load_jump_back_helper && !do_kexec_fallback)
 			die("--load-jump-back-helper not supported with kexec_file_load\n");