[BUGFIX][PATCH 1/4] dbg_rw_guest_mem: need to call put_gfn in error path.

[Don Slutz <dslutz@verizon.com>]

Using a 1G hvm domU (in grub) and gdbsx:

(gdb) set arch i8086
warning: A handler for the OS ABI "GNU/Linux" is not built into this configuration
of GDB.  Attempting to continue with the default i8086 settings.

The target architecture is assumed to be i8086
(gdb) target remote localhost:9999
Remote debugging using localhost:9999
Remote debugging from host 127.0.0.1
0x0000d475 in ?? ()
(gdb) x/1xh 0x6ae9168b

Will reproduce this bug.

With a debug=y build you will get:

Assertion '!preempt_count()' failed at preempt.c:37

For a debug=n build you will get a dom0 VCPU hung (at some point) in:

         [ffff82c4c0126eec] _write_lock+0x3c/0x50
          ffff82c4c01e43a0  __get_gfn_type_access+0x150/0x230
          ffff82c4c0158885  dbg_rw_mem+0x115/0x360
          ffff82c4c0158fc8  arch_do_domctl+0x4b8/0x22f0
          ffff82c4c01709ed  get_page+0x2d/0x100
          ffff82c4c01031aa  do_domctl+0x2ba/0x11e0
          ffff82c4c0179662  do_mmuext_op+0x8d2/0x1b20
          ffff82c4c0183598  __update_vcpu_system_time+0x288/0x340
          ffff82c4c015c719  continue_nonidle_domain+0x9/0x30
          ffff82c4c012938b  add_entry+0x4b/0xb0
          ffff82c4c02223f9  syscall_enter+0xa9/0xae

And gdb output:

(gdb) x/1xh 0x6ae9168b
0x6ae9168b:     0x3024
(gdb) x/1xh 0x6ae9168b
0x6ae9168b:     Ignoring packet error, continuing...
Reply contains invalid hex digit 116

The 1st one worked because the p2m.lock is recursive and the PCPU
had not yet changed.

crash reports (for example):

crash> mm_rwlock_t 0xffff83083f913010
struct mm_rwlock_t {
  lock = {
    raw = {
      lock = 2147483647
    },
    debug = {<No data fields>}
  },
  unlock_level = 0,
  recurse_count = 1,
  locker = 1,
  locker_function = 0xffff82c4c022c640 <__func__.13514> "__get_gfn_type_access"
}

Signed-off-by: Don Slutz <dslutz@verizon.com>
---
 xen/arch/x86/debug.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/debug.c b/xen/arch/x86/debug.c
index 3e21ca8..eceb805 100644
--- a/xen/arch/x86/debug.c
+++ b/xen/arch/x86/debug.c
@@ -161,8 +161,11 @@ dbg_rw_guest_mem(dbgva_t addr, dbgbyte_t *buf, int len, struct domain *dp,
         mfn = (has_hvm_container_domain(dp)
                ? dbg_hvm_va2mfn(addr, dp, toaddr, &gfn)
                : dbg_pv_va2mfn(addr, dp, pgd3));
-        if ( mfn == INVALID_MFN ) 
+        if ( mfn == INVALID_MFN ) {
+            if ( gfn != INVALID_GFN )
+                put_gfn(dp, gfn);
             break;
+        }
 
         va = map_domain_page(mfn);
         va = va + (addr & (PAGE_SIZE-1));
-- 
1.8.4