[Qemu-devel] [PATCH 4/4] pseries: Remove unnecessary locking from PAPR hash table hcalls

[David Gibson]

In the paravirtualized environment provided by PAPR, there is a standard
locking scheme so that hypercalls updating the hash page table from
different guest threads don't corrupt the haah table state.  We implement
this HVLOCK bit in out page table hypercalls.  However, it is not necessary
in our case, since the hypercalls all run in the qemu environment under the
big qemu lock.

Therefore, this patch removes the locking code.  This has the additional
advantage of freeing up a hash PTE bit which will be useful for migration
support.

Signed-off-by: David Gibson <address@hidden>
---
 hw/spapr_hcall.c |   42 ++++--------------------------------------
 1 file changed, 4 insertions(+), 38 deletions(-)

diff --git a/hw/spapr_hcall.c b/hw/spapr_hcall.c
index 2df94d1..826ca67 100644
--- a/hw/spapr_hcall.c
+++ b/hw/spapr_hcall.c
@@ -39,22 +39,6 @@
 #define HPTE_V_1TB_SEG          0x4000000000000000ULL
 #define HPTE_V_VRMA_MASK        0x4001ffffff000000ULL
 
-#define HPTE_V_HVLOCK           0x40ULL
-
-static inline int lock_hpte(void *hpte, target_ulong bits)
-{
-    uint64_t pteh;
-
-    pteh = ldq_p(hpte);
-
-    /* We're protected by qemu's global lock here */
-    if (pteh & bits) {
-        return 0;
-    }
-    stq_p(hpte, pteh | HPTE_V_HVLOCK);
-    return 1;
-}
-
 static target_ulong compute_tlbie_rb(target_ulong v, target_ulong r,
                                      target_ulong pte_index)
 {
@@ -151,8 +135,7 @@ static target_ulong h_enter(CPUPPCState *env, 
sPAPREnvironment *spapr,
             if (i == 8) {
                 return H_PTEG_FULL;
             }
-            if (((ldq_p(hpte) & HPTE_V_VALID) == 0) &&
-                lock_hpte(hpte, HPTE_V_HVLOCK | HPTE_V_VALID)) {
+            if ((ldq_p(hpte) & HPTE_V_VALID) == 0) {
                 break;
             }
             hpte += HASH_PTE_SIZE_64;
@@ -160,7 +143,7 @@ static target_ulong h_enter(CPUPPCState *env, 
sPAPREnvironment *spapr,
     } else {
         i = 0;
         hpte = env->external_htab + (pte_index * HASH_PTE_SIZE_64);
-        if (!lock_hpte(hpte, HPTE_V_HVLOCK | HPTE_V_VALID)) {
+        if (ldq_p(hpte) & HPTE_V_VALID) {
             return H_PTEG_FULL;
         }
     }
@@ -168,7 +151,6 @@ static target_ulong h_enter(CPUPPCState *env, 
sPAPREnvironment *spapr,
     /* eieio();  FIXME: need some sort of barrier for smp? */
     stq_p(hpte, pteh);
 
-    assert(!(ldq_p(hpte) & HPTE_V_HVLOCK));
     args[0] = pte_index + i;
     return H_SUCCESS;
 }
@@ -193,11 +175,6 @@ static target_ulong remove_hpte(CPUPPCState *env, 
target_ulong ptex,
     }
 
     hpte = env->external_htab + (ptex * HASH_PTE_SIZE_64);
-    while (!lock_hpte(hpte, HPTE_V_HVLOCK)) {
-        /* We have no real concurrency in qemu soft-emulation, so we
-         * will never actually have a contested lock */
-        assert(0);
-    }
 
     v = ldq_p(hpte);
     r = ldq_p(hpte + (HASH_PTE_SIZE_64/2));
@@ -205,16 +182,13 @@ static target_ulong remove_hpte(CPUPPCState *env, 
target_ulong ptex,
     if ((v & HPTE_V_VALID) == 0 ||
         ((flags & H_AVPN) && (v & ~0x7fULL) != avpn) ||
         ((flags & H_ANDCOND) && (v & avpn) != 0)) {
-        stq_p(hpte, v & ~HPTE_V_HVLOCK);
-        assert(!(ldq_p(hpte) & HPTE_V_HVLOCK));
         return REMOVE_NOT_FOUND;
     }
-    *vp = v & ~HPTE_V_HVLOCK;
+    *vp = v;
     *rp = r;
     stq_p(hpte, 0);
     rb = compute_tlbie_rb(v, r, ptex);
     ppc_tlb_invalidate_one(env, rb);
-    assert(!(ldq_p(hpte) & HPTE_V_HVLOCK));
     return REMOVE_SUCCESS;
 }
 
@@ -324,19 +298,12 @@ static target_ulong h_protect(CPUPPCState *env, 
sPAPREnvironment *spapr,
     }
 
     hpte = env->external_htab + (pte_index * HASH_PTE_SIZE_64);
-    while (!lock_hpte(hpte, HPTE_V_HVLOCK)) {
-        /* We have no real concurrency in qemu soft-emulation, so we
-         * will never actually have a contested lock */
-        assert(0);
-    }
 
     v = ldq_p(hpte);
     r = ldq_p(hpte + (HASH_PTE_SIZE_64/2));
 
     if ((v & HPTE_V_VALID) == 0 ||
         ((flags & H_AVPN) && (v & ~0x7fULL) != avpn)) {
-        stq_p(hpte, v & ~HPTE_V_HVLOCK);
-        assert(!(ldq_p(hpte) & HPTE_V_HVLOCK));
         return H_NOT_FOUND;
     }
 
@@ -350,8 +317,7 @@ static target_ulong h_protect(CPUPPCState *env, 
sPAPREnvironment *spapr,
     ppc_tlb_invalidate_one(env, rb);
     stq_p(hpte + (HASH_PTE_SIZE_64/2), r);
     /* Don't need a memory barrier, due to qemu's global lock */
-    stq_p(hpte, v & ~HPTE_V_HVLOCK);
-    assert(!(ldq_p(hpte) & HPTE_V_HVLOCK));
+    stq_p(hpte, v);
     return H_SUCCESS;
 }
 
-- 
1.7.10.4