[PATCH v2 04/39] xen/riscv: introduce csr_read64()

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

csr_write64() already hides the RV32 split of a 64-bit CSR into a low and
a high half; add the read counterpart.

Reading the two halves isn't simply the mirror of writing them. A CSR which
hardware increments can carry from the low half into the high one between
the two reads, so a plain pair of reads can produce a value the CSR never
held. Therefore the high half is re-read and the sequence retried if it
changed in the meantime.

Use it for CSR_TIME, which is exactly such a counter, and widen cycles_t to
uint64_t. Otherwise get_cycles() would still truncate the time counter to
32 bits on RV32.

Fixes: a541ddadec0a ("xen/riscv: introduce time.h")
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
---
Changes in v2:
 - New patch.
---
---
 xen/arch/riscv/include/asm/csr.h  | 24 ++++++++++++++++++++++++
 xen/arch/riscv/include/asm/time.h |  4 ++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/xen/arch/riscv/include/asm/csr.h b/xen/arch/riscv/include/asm/csr.h
index 888d6a2a86d6..a5cdd6f99c8e 100644
--- a/xen/arch/riscv/include/asm/csr.h
+++ b/xen/arch/riscv/include/asm/csr.h
@@ -39,12 +39,36 @@
     csr_write(csr, v_);             \
     csr_write(csr ## H, v_ >> 32);  \
 })
+
+/*
+ * The two halves are read by separate instructions, so a CSR which hardware
+ * increments can carry from the low half into the high one in between,
+ * yielding a value the CSR never held. Re-read the high half and retry the
+ * sequence if it changed.
+ */
+#define csr_read64(csr)                         \
+({                                              \
+    uint32_t hi_, lo_;                          \
+                                                \
+    do {                                        \
+        hi_ = csr_read(csr ## H);               \
+        lo_ = csr_read(csr);                    \
+    } while ( hi_ != csr_read(csr ## H) );      \
+                                                \
+    ((uint64_t)hi_ << 32) | lo_;                \
+})
 #else
 #define csr_write64(csr, val)       \
 ({                                  \
     csr_write(csr, val);            \
     (void)csr ## H;                 \
 })
+
+#define csr_read64(csr)             \
+({                                  \
+    (void)csr ## H;                 \
+    csr_read(csr);                  \
+})
 #endif
 
 #define csr_swap(csr, val)                                      \
diff --git a/xen/arch/riscv/include/asm/time.h 
b/xen/arch/riscv/include/asm/time.h
index 4d68900151a7..ec771c3fe80f 100644
--- a/xen/arch/riscv/include/asm/time.h
+++ b/xen/arch/riscv/include/asm/time.h
@@ -18,11 +18,11 @@ static inline void force_update_vcpu_system_time(struct 
vcpu *v)
     BUG_ON("unimplemented");
 }
 
-typedef unsigned long cycles_t;
+typedef uint64_t cycles_t;
 
 static inline cycles_t get_cycles(void)
 {
-    return csr_read(CSR_TIME);
+    return csr_read64(CSR_TIME);
 }
 
 void preinit_xen_time(void);
-- 
2.55.0