[Qemu-devel] [PATCH 3/3] ppc: Fix radix RC updates

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

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 <benh@kernel.crashing.org>
---
 target/ppc/cpu.h         |  1 +
 target/ppc/mmu-radix64.c | 70 +++++++++++++++++++++++++++++++++-------
 2 files changed, 59 insertions(+), 12 deletions(-)

diff --git a/target/ppc/cpu.h b/target/ppc/cpu.h
index ab68abe8a2..afdef2af2f 100644
--- a/target/ppc/cpu.h
+++ b/target/ppc/cpu.h
@@ -493,6 +493,7 @@ struct ppc_slb_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 ab76cbc835..dba95aabdc 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)
 {
@@ -120,11 +129,18 @@ 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;
@@ -133,17 +149,38 @@ 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 {
-        /*
-         * Treat the page as read-only for now, so that a later write
-         * will pass through this function again to set the C bit.
-         */
-        *prot &= ~PAGE_WRITE;
     }
+    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;
+        }
 
-    if (pte ^ npte) { /* If pte has changed then write it back */
-        stq_phys(cs->as, pte_addr, npte);
+        /* Do we need to support this case where PTEs aren't in RAM ?
+         *
+         * For now fallback to non-atomic case
+         */
     }
+#endif
+
+    stq_phys(cs->as, pte_addr, npte);
+    return npte;
 }
 
 static uint64_t ppc_radix64_walk_tree(PowerPCCPU *cpu, vaddr eaddr,
@@ -234,6 +271,7 @@ 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);
@@ -244,8 +282,16 @@ 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;
-- 
2.19.2