[RFC PATCH 1/1] arm/ptw: Handle atomic updates of page tables entries in MMIO during PTW.

[Jonathan Cameron via <qemu-devel@nongnu.org>]

I'm far from confident this handling here is correct. Hence
RFC.  In particular not sure on what locks I should hold for this
to be even moderately safe.

The function already appears to be inconsistent in what it returns
as the CONFIG_ATOMIC64 block returns the endian converted 'eventual'
value of the cmpxchg whereas the TCG_OVERSIZED_GUEST case returns
the previous value.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
---
 target/arm/ptw.c | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/target/arm/ptw.c b/target/arm/ptw.c
index 5eb3577bcd..37ddb4a4db 100644
--- a/target/arm/ptw.c
+++ b/target/arm/ptw.c
@@ -711,8 +711,38 @@ static uint64_t arm_casq_ptw(CPUARMState *env, uint64_t old_val,
     void *host = ptw->out_host;
 
     if (unlikely(!host)) {
-        fi->type = ARMFault_UnsuppAtomicUpdate;
-        return 0;
+        /* Can I do a load and store via the physical address */
+
+        bool locked = bql_locked();
+        if (!locked) {
+            bql_lock();
+        }
+        /* Page table in MMIO Memory Region */
+        if (ptw->out_be) {
+            old_val = cpu_to_be64(old_val);
+            new_val = cpu_to_be64(new_val);
+            cpu_physical_memory_read(ptw->out_phys, &cur_val, 8);
+            if (cur_val == old_val) {
+                cpu_physical_memory_write(ptw->out_phys, &new_val, 8);
+                cur_val = be64_to_cpu(new_val);
+            } else {
+                cur_val = be64_to_cpu(cur_val);
+            }
+        } else {
+            old_val = cpu_to_le64(old_val);
+            new_val = cpu_to_le64(new_val);
+            cpu_physical_memory_read(ptw->out_phys, &cur_val, 8);
+            if (cur_val == old_val) {
+                cpu_physical_memory_write(ptw->out_phys, &new_val, 8);
+                cur_val = le64_to_cpu(new_val);
+            } else {
+                cur_val = le64_to_cpu(cur_val);
+            }
+        }
+        if (!locked) {
+            bql_unlock();
+        }
+        return cur_val;
     }
 
     /*
-- 
2.39.2