[Minios-devel] [UNIKRAFT/UNIKRAFT PATCH 2/2] Temporary workaround to have atomic operations working on ARM64 baremetal

[Santiago Pagani <santiago.pagani@xxxxxxxxx>]

At the moment atomic operations are working on ARM64 for KVM, however,
they are not working on baremetal on ARM64 (i.e., on the Raspberry Pi 3B+).
This can be most likely be solved by enabling the
caches and setting up a compatible memory configuration.
In the mean time, this compile guard allows us to have functionally-equivalent
non-atomic implementations of the gcc builtins working on baremetal on ARM64.
Once the issue is fixed, then this workaround can be removed.

Signed-off-by: Santiago Pagani <santiagopagani@xxxxxxxxx>
---
 include/uk/arch/atomic.h | 47 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 47 insertions(+)

diff --git a/include/uk/arch/atomic.h b/include/uk/arch/atomic.h
index 6fae1762..0c76b33a 100644
--- a/include/uk/arch/atomic.h
+++ b/include/uk/arch/atomic.h
@@ -41,20 +41,37 @@ extern "C" {
 /**
  * Perform a atomic load operation.
  */
+// #TODO: This (and the other compile guards in this file for 
CONFIG_PLAT_RASPI) is a workaround until we get proper
+// atomic operations working on ARM64 baremetal.
+// Mainly, whenever we try to execute a ldaxr or stlxr instruction on 
baremetal with the current memory configuration,
+// the CPU simply hangs. This should be fixed by enabling the caches and/or 
setting up a compatible memory configuration.
+#if CONFIG_PLAT_RASPI
+#define ukarch_load_n(src)             (*(src))
+#else
 #define ukarch_load_n(src) \
        __atomic_load_n(src, __ATOMIC_SEQ_CST)
+#endif
+
 
 /**
  * Perform a atomic store operation.
  */
+#if CONFIG_PLAT_RASPI
+#define ukarch_store_n(src, value)     ((*(src)) = value)
+#else
 #define ukarch_store_n(src, value) \
        __atomic_store_n(src, value, __ATOMIC_SEQ_CST)
+#endif
 
 /**
  * Perform a atomic fetch and add operation.
  */
+#if CONFIG_PLAT_RASPI
+#define ukarch_fetch_add(src, value)   ((*(src))++)
+#else
 #define ukarch_fetch_add(src, value) \
        __atomic_fetch_add(src, value, __ATOMIC_SEQ_CST)
+#endif
 
 /**
  * Perform a atomic increment/decrement operation and return the
@@ -62,20 +79,49 @@ extern "C" {
  */
 #define ukarch_inc(src) \
        ukarch_fetch_add(src, 1)
+#if CONFIG_PLAT_RASPI
+#define ukarch_dec(src)                ((*(src))--)
+#else
 #define ukarch_dec(src) \
        __atomic_fetch_sub(src, 1, __ATOMIC_SEQ_CST)
+#endif
+
 /**
  * Writes *src into *dst, and returns the previous contents of *dst.
  */
+#if CONFIG_PLAT_RASPI
+#define ukarch_exchange(dst, src)              \
+       ({                                                                      
\
+               __typeof__(*dst) prev = *dst;   \
+               *dst = *src;                                    \
+               prev;                                                   \
+       })
+#else
 #define ukarch_exchange(dst, src) \
        __atomic_exchange(dst, src, __ATOMIC_SEQ_CST)
+#endif
 
 /**
  * Writes v into *dst, and returns the previous contents of *dst.
  */
+#if CONFIG_PLAT_RASPI
+#define ukarch_exchange_n(dst, v)              \
+       ({                                                                      
\
+               __typeof__(*dst) prev = *dst;   \
+               *dst = v;                                               \
+               prev;                                                   \
+       })
+#else
 #define ukarch_exchange_n(dst, v) \
        __atomic_exchange_n(dst, v, __ATOMIC_SEQ_CST)
+#endif
 
+#if CONFIG_PLAT_RASPI
+#define ukarch_compare_exchange_sync(ptr, old, new)                            
                \
+       ({                                                                      
                                                                \
+               (*ptr == old) ? (*ptr = new) : old;                             
                                \
+       })
+#else
 #define ukarch_compare_exchange_sync(ptr, old, new)                            
\
        ({                                                                     \
                __typeof__(*ptr) stored = old;                                 \
@@ -84,6 +130,7 @@ extern "C" {
                    ? new                                                      \
                    : old;                                                     \
        })
+#endif
 
 #ifdef __cplusplus
 }
-- 
2.17.1


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