[Qemu-devel] [PATCH 6/6] ppc: Fix radix RC updates

[Cédric Le Goater]

From: Benjamin Herrenschmidt <address@hidden>

They should be atomic for MTTCG. Note: a real POWER9 core doesn't actually
implement atomic PTE updates, it always fault for SW to handle it. Only
the nest MMU (used by some accelerator devices and GPUs) implements
those HW updates.

However, the architecture does allow the core to do it, and doing so
in TCG is faster than letting the guest do it.

Signed-off-by: Benjamin Herrenschmidt <address@hidden>
Signed-off-by: Cédric Le Goater <address@hidden>
---
 target/ppc/cpu.h         |  1 +
 target/ppc/mmu-radix64.c | 70 +++++++++++++++++++++++++++++++++-------
 2 files changed, 60 insertions(+), 11 deletions(-)

diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
index a4420636dde0..5b4249ebdcbf 100644
--- a/target/ppc/cpu.h
+++ b/target/ppc/cpu.h
@@ -503,6 +503,7 @@ typedef struct ppc_v3_pate_t {
 #define DSISR_AMR                0x00200000
 /* Unsupported Radix Tree Configuration */
 #define DSISR_R_BADCONFIG        0x00080000
+#define DSISR_ATOMIC_RC          0x00040000
 
 /* SRR1 error code fields */
 
diff --git a/target/ppc/mmu-radix64.c b/target/ppc/mmu-radix64.c
index cac076ee92d9..458085d79ee0 100644
--- a/target/ppc/mmu-radix64.c
+++ b/target/ppc/mmu-radix64.c
@@ -28,6 +28,15 @@
 #include "mmu-radix64.h"
 #include "mmu-book3s-v3.h"
 
+static inline bool ppc_radix64_hw_rc_updates(CPUPPCState *env)
+{
+#ifdef CONFIG_ATOMIC64
+    return true;
+#else
+    return !qemu_tcg_mttcg_enabled();
+#endif
+}
+
 static bool ppc_radix64_get_fully_qualified_addr(CPUPPCState *env, vaddr eaddr,
                                                  uint64_t *lpid, uint64_t *pid)
 {
@@ -136,11 +145,19 @@ static bool ppc_radix64_check_prot(PowerPCCPU *cpu, int 
rwx, uint64_t pte,
         return true;
     }
 
+    /* Check RC bits if necessary */
+    if (!ppc_radix64_hw_rc_updates(env)) {
+        if (!(pte & R_PTE_R) || ((rwx == 1) && !(pte & R_PTE_C))) {
+            *fault_cause |= DSISR_ATOMIC_RC;
+            return true;
+        }
+    }
+
     return false;
 }
 
-static void ppc_radix64_set_rc(PowerPCCPU *cpu, int rwx, uint64_t pte,
-                               hwaddr pte_addr, int *prot)
+static uint64_t ppc_radix64_set_rc(PowerPCCPU *cpu, int rwx, uint64_t pte,
+                                   hwaddr pte_addr)
 {
     CPUState *cs = CPU(cpu);
     uint64_t npte;
@@ -149,17 +166,39 @@ static void ppc_radix64_set_rc(PowerPCCPU *cpu, int rwx, 
uint64_t pte,
 
     if (rwx == 1) { /* Store/Write */
         npte |= R_PTE_C; /* Set change bit */
-    } else {
+    }
+    if (pte == npte) {
+        return pte;
+    }
+
+#ifdef CONFIG_ATOMIC64
+    if (qemu_tcg_mttcg_enabled()) {
+        uint64_t old_be = cpu_to_be32(pte);
+        uint64_t new_be = cpu_to_be32(npte);
+        MemTxResult result;
+        uint64_t old_ret;
+
+        old_ret = address_space_cmpxchgq_notdirty(cs->as, pte_addr,
+                                                  old_be, new_be,
+                                                  MEMTXATTRS_UNSPECIFIED,
+                                                  &result);
+        if (result == MEMTX_OK) {
+            if (old_ret != old_be && old_ret != new_be) {
+                return 0;
+            }
+            return npte;
+        }
+
         /*
-         * Treat the page as read-only for now, so that a later write
-         * will pass through this function again to set the C bit.
+         * Do we need to support this case where PTEs aren't in RAM ?
+         *
+         * For now fallback to non-atomic case
          */
-        *prot &= ~PAGE_WRITE;
     }
+#endif
 
-    if (pte ^ npte) { /* If pte has changed then write it back */
-        stq_phys(cs->as, pte_addr, npte);
-    }
+    stq_phys(cs->as, pte_addr, npte);
+    return npte;
 }
 
 static uint64_t ppc_radix64_walk_tree(PowerPCCPU *cpu, vaddr eaddr,
@@ -288,6 +327,7 @@ int ppc_radix64_handle_mmu_fault(PowerPCCPU *cpu, vaddr 
eaddr, int rwx,
 
     /* Walk Radix Tree from Process Table Entry to Convert EA to RA */
     page_size = PRTBE_R_GET_RTS(prtbe0);
+ restart:
     pte = ppc_radix64_walk_tree(cpu, eaddr & R_EADDR_MASK,
                                 prtbe0 & PRTBE_R_RPDB, prtbe0 & PRTBE_R_RPDS,
                                 &raddr, &page_size, &fault_cause, &pte_addr);
@@ -298,8 +338,16 @@ int ppc_radix64_handle_mmu_fault(PowerPCCPU *cpu, vaddr 
eaddr, int rwx,
     }
 
     /* Update Reference and Change Bits */
-    ppc_radix64_set_rc(cpu, rwx, pte, pte_addr, &prot);
-
+    if (ppc_radix64_hw_rc_updates(env)) {
+        pte = ppc_radix64_set_rc(cpu, rwx, pte, pte_addr);
+        if (!pte) {
+            goto restart;
+        }
+    }
+    /* If the page doesn't have C, treat it as read only */
+    if (!(pte & R_PTE_C)) {
+        prot &= ~PAGE_WRITE;
+    }
     tlb_set_page(cs, eaddr & TARGET_PAGE_MASK, raddr & TARGET_PAGE_MASK,
                  prot, mmu_idx, 1UL << page_size);
     return 0;
-- 
2.20.1