[Qemu-devel] [PATCH v2] target-s390x: fix LOAD MULTIPLE instruction on page boundary

[Aurelien Jarno]

When consecutive memory locations are on page boundary a page fault
might occur when using the LOAD MULTIPLE instruction. In that case real
hardware doesn't load any register.

This is an important detail in case the base register is in the list
of registers to be loaded. If a page fault occurs this register might be
overwritten and when the instruction is later restarted the wrong
base register value is useD.

Fix this by first loading the first and last value from memory, hence
triggering all possible page faults, and then the remaining registers.

This fixes random segmentation faults seen in the guest.

Cc: Alexander Graf <address@hidden>
Cc: Richard Henderson <address@hidden>
Signed-off-by: Aurelien Jarno <address@hidden>
---
 target-s390x/translate.c | 128 ++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 99 insertions(+), 29 deletions(-)

Changes v1->v2:
- Do the load in two steps: first and last registers, and then the remaining 
ones

diff --git a/target-s390x/translate.c b/target-s390x/translate.c
index 1819217..23c78e7 100644
--- a/target-s390x/translate.c
+++ b/target-s390x/translate.c
@@ -2434,21 +2434,45 @@ static ExitStatus op_lm32(DisasContext *s, DisasOps *o)
 {
     int r1 = get_field(s->fields, r1);
     int r3 = get_field(s->fields, r3);
-    TCGv_i64 t = tcg_temp_new_i64();
-    TCGv_i64 t4 = tcg_const_i64(4);
+    TCGv_i64 t1, t2;
 
-    while (1) {
-        tcg_gen_qemu_ld32u(t, o->in2, get_mem_index(s));
-        store_reg32_i64(r1, t);
-        if (r1 == r3) {
-            break;
-        }
-        tcg_gen_add_i64(o->in2, o->in2, t4);
+    /* Only one register to read. */
+    t1 = tcg_temp_new_i64();
+    if (unlikely(r1 == r3)) {
+        tcg_gen_qemu_ld32u(t1, o->in2, get_mem_index(s));
+        store_reg32_i64(r1, t1);
+        tcg_temp_free(t1);
+        return NO_EXIT;
+    }
+
+    /* First load the values of the first and last registers to trigger
+       possible page faults. */
+    t2 = tcg_temp_new_i64();
+    tcg_gen_qemu_ld32u(t1, o->in2, get_mem_index(s));
+    tcg_gen_addi_i64(t2, o->in2, 4 * ((r3 - r1) & 15));
+    tcg_gen_qemu_ld32u(t2, t2, get_mem_index(s));
+    store_reg32_i64(r1, t1);
+    store_reg32_i64(r3, t2);
+
+    /* Only two registers to read. */
+    if (((r1 + 1) & 15) == r3) {
+        tcg_temp_free(t2);
+        tcg_temp_free(t1);
+        return NO_EXIT;
+    }
+
+    /* Then load the remaining registers. Page fault can't occur. */
+    r3 = (r3 - 1) & 15;
+    tcg_gen_movi_i64(t2, 4);
+    while (r1 != r3) {
         r1 = (r1 + 1) & 15;
+        tcg_gen_add_i64(o->in2, o->in2, t2);
+        tcg_gen_qemu_ld32u(t1, o->in2, get_mem_index(s));
+        store_reg32_i64(r1, t1);
     }
+    tcg_temp_free(t2);
+    tcg_temp_free(t1);
 
-    tcg_temp_free_i64(t);
-    tcg_temp_free_i64(t4);
     return NO_EXIT;
 }
 
@@ -2456,21 +2480,45 @@ static ExitStatus op_lmh(DisasContext *s, DisasOps *o)
 {
     int r1 = get_field(s->fields, r1);
     int r3 = get_field(s->fields, r3);
-    TCGv_i64 t = tcg_temp_new_i64();
-    TCGv_i64 t4 = tcg_const_i64(4);
+    TCGv_i64 t1, t2;
 
-    while (1) {
-        tcg_gen_qemu_ld32u(t, o->in2, get_mem_index(s));
-        store_reg32h_i64(r1, t);
-        if (r1 == r3) {
-            break;
-        }
-        tcg_gen_add_i64(o->in2, o->in2, t4);
+    /* Only one register to read. */
+    t1 = tcg_temp_new_i64();
+    if (unlikely(r1 == r3)) {
+        tcg_gen_qemu_ld32u(t1, o->in2, get_mem_index(s));
+        store_reg32h_i64(r1, t1);
+        tcg_temp_free(t1);
+        return NO_EXIT;
+    }
+
+    /* First load the values of the first and last registers to trigger
+       possible page faults. */
+    t2 = tcg_temp_new_i64();
+    tcg_gen_qemu_ld32u(t1, o->in2, get_mem_index(s));
+    tcg_gen_addi_i64(t2, o->in2, 4 * ((r3 - r1) & 15));
+    tcg_gen_qemu_ld32u(t2, t2, get_mem_index(s));
+    store_reg32h_i64(r1, t1);
+    store_reg32h_i64(r3, t2);
+
+    /* Only two registers to read. */
+    if (((r1 + 1) & 15) == r3) {
+        tcg_temp_free(t2);
+        tcg_temp_free(t1);
+        return NO_EXIT;
+    }
+
+    /* Then load the remaining registers. Page fault can't occur. */
+    r3 = (r3 - 1) & 15;
+    tcg_gen_movi_i64(t2, 4);
+    while (r1 != r3) {
         r1 = (r1 + 1) & 15;
+        tcg_gen_add_i64(o->in2, o->in2, t2);
+        tcg_gen_qemu_ld32u(t1, o->in2, get_mem_index(s));
+        store_reg32h_i64(r1, t1);
     }
+    tcg_temp_free(t2);
+    tcg_temp_free(t1);
 
-    tcg_temp_free_i64(t);
-    tcg_temp_free_i64(t4);
     return NO_EXIT;
 }
 
@@ -2478,18 +2526,40 @@ static ExitStatus op_lm64(DisasContext *s, DisasOps *o)
 {
     int r1 = get_field(s->fields, r1);
     int r3 = get_field(s->fields, r3);
-    TCGv_i64 t8 = tcg_const_i64(8);
+    TCGv_i64 t1, t2;
 
-    while (1) {
+    /* Only one register to read. */
+    if (unlikely(r1 == r3)) {
         tcg_gen_qemu_ld64(regs[r1], o->in2, get_mem_index(s));
-        if (r1 == r3) {
-            break;
-        }
-        tcg_gen_add_i64(o->in2, o->in2, t8);
+        return NO_EXIT;
+    }
+
+    /* First load the values of the first and last registers to trigger
+       possible page faults. */
+    t1 = tcg_temp_new_i64();
+    t2 = tcg_temp_new_i64();
+    tcg_gen_qemu_ld64(t1, o->in2, get_mem_index(s));
+    tcg_gen_addi_i64(t2, o->in2, 8 * ((r3 - r1) & 15));
+    tcg_gen_qemu_ld64(regs[r3], t2, get_mem_index(s));
+    tcg_gen_mov_i64(regs[r1], t1);
+    tcg_temp_free(t2);
+
+    /* Only two registers to read. */
+    if (((r1 + 1) & 15) == r3) {
+        tcg_temp_free(t1);
+        return NO_EXIT;
+    }
+
+    /* Then load the remaining registers. Page fault can't occur. */
+    r3 = (r3 - 1) & 15;
+    tcg_gen_movi_i64(t1, 8);
+    while (r1 != r3) {
         r1 = (r1 + 1) & 15;
+        tcg_gen_add_i64(o->in2, o->in2, t1);
+        tcg_gen_qemu_ld64(regs[r1], o->in2, get_mem_index(s));
     }
+    tcg_temp_free(t1);
 
-    tcg_temp_free_i64(t8);
     return NO_EXIT;
 }
 
-- 
2.1.4