[Crash-utility] Fix pvops Xen detection for kernels >= v4.20

[Petr Tesařík]

Kernel commit 5c83511bdb9832c86be20fb86b783356e2f58062 removed 
pv_init_ops, and later commit 054ac8ad5ebe4a69e1f0e842483821ddbe560121 
removed the Xen-specific paravirt patch function. As a result, pvops Xen 
dumps are no longer recognized as Xen dumps, and virtual-to-physical 
translation fails.

Use the value of xen_start_info to determine whether the kernel is 
running in Xen PV mode. This pointer is set during the initialization of 
a PV domain. Kudos to Juergen Gross, who suggested this check.

Signed-off-by: Petr Tesarik <ptesarik at suse.com>
---
  kernel.c |   34 ++++++++++++++++++++++++++++------
  1 file changed, 28 insertions(+), 6 deletions(-)

diff --git a/kernel.c b/kernel.c
index e123f76..36fdea2 100644
--- a/kernel.c
+++ b/kernel.c
@@ -95,6 +95,7 @@ static ulong __dump_audit(char *);
  static void dump_audit(void);
  static char *vmcoreinfo_read_string(const char *);
  static void check_vmcoreinfo(void);
+static int is_pvops_xen(void);


  /*
@@ -109,7 +110,6 @@ kernel_init()
  	char *rqstruct;
  	char *rq_timestamp_name = NULL;
  	char *irq_desc_type_name;	
-	ulong pv_init_ops;
  	struct gnu_request req;

  	if (pc->flags & KERNEL_DEBUG_QUERY)
@@ -169,11 +169,7 @@ kernel_init()
                         	error(FATAL, "cannot malloc m2p page.");
  	}

-	if (PVOPS() && symbol_exists("pv_init_ops") &&
-	    readmem(symbol_value("pv_init_ops"), KVADDR, &pv_init_ops,
-	    sizeof(void *), "pv_init_ops", RETURN_ON_ERROR) &&
-	    ((p1 = value_symbol(pv_init_ops)) &&
-	    (STREQ(p1, "xen_patch") || STREQ(p1, "paravirt_patch_default")))) {
+	if (is_pvops_xen()) {
  		kt->flags |= ARCH_XEN | ARCH_PVOPS_XEN;
  		kt->xen_flags |= WRITABLE_PAGE_TABLES;
  		if (machine_type("X86"))
@@ -10709,6 +10705,32 @@ paravirt_init(void)
  	}
  }

+static int
+is_pvops_xen(void)
+{
+	ulong addr;
+	char *sym;
+
+	if (!PVOPS())
+		return FALSE;
+
+	if (symbol_exists("pv_init_ops") &&
+	    readmem(symbol_value("pv_init_ops"), KVADDR, &addr,
+	    sizeof(void *), "pv_init_ops", RETURN_ON_ERROR) &&
+	    (sym = value_symbol(addr)) &&
+	    (STREQ(sym, "xen_patch") ||
+	     STREQ(sym, "paravirt_patch_default")))
+		return TRUE;
+
+	if (symbol_exists("xen_start_info") &&
+	    readmem(symbol_value("xen_start_info"), KVADDR, &addr,
+	    sizeof(void *), "xen_start_info", RETURN_ON_ERROR) &&
+	    addr != 0)
+		return TRUE;
+
+	return FALSE;
+}
+
  /*
   *  Get the kernel's xtime timespec from its relevant location.
   */