[PATCH] target/ppc: Fix mtmsr(d) L=1 variant that loses interrupts

[Nicholas Piggin]

If mtmsr L=1 sets MSR[EE] while there is a maskable exception pending,
it does not cause an interrupt. This causes the test case to hang:

https://lists.gnu.org/archive/html/qemu-ppc/2019-10/msg00826.html

More recently, Linux reduced the occurance of operations (e.g., rfi)
which stop translation and allow pending interrupts to be processed.
This started causing hangs in Linux boot in long-running kernel tests,
running with '-d int' shows the decrementer stops firing despite DEC
wrapping and MSR[EE]=1.

https://lists.ozlabs.org/pipermail/linuxppc-dev/2020-April/208301.html

The cause is the broken mtmsr L=1 behaviour, which is contrary to the
architecture. From Power ISA v3.0B, p.977, Move To Machine State Register,
Programming Note states:

    If MSR[EE]=0 and an External, Decrementer, or Performance Monitor
    exception is pending, executing an mtmsrd instruction that sets
    MSR[EE] to 1 will cause the interrupt to occur before the next
    instruction is executed, if no higher priority exception exists

Fix this by handling L=1 exactly the same way as L=0, modulo the MSR
bits altered.

The confusion arises from L=0 being "context synchronizing" whereas L=1
is "execution synchronizing", which is a weaker semantic. However this
is not a relaxation of the requirement that these exceptions cause
interrupts when MSR[EE]=1 (e.g., when mtmsr executes to completion as
TCG is doing here), rather it specifies how a pipelined processor can
have multiple instructions in flight where one may influence how another
behaves.

Cc: qemu-stable at nongnu.org
Reported-by: Anton Blanchard <anton at ozlabs.org>
Reported-by: Nathan Chancellor <natechancellor at gmail.com>
Tested-by: Nathan Chancellor <natechancellor at gmail.com>
Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
---
Thanks very much to Nathan for reporting and testing it, I added his
Tested-by tag despite a more polished patch, as the the basics are 
still the same (and still fixes his test case here).

This bug possibly goes back to early v2.04 / mtmsrd L=1 support around
2007, and the code has been changed several times since then so may
require some backporting.

32-bit / mtmsr untested at the moment, I don't have an environment
handy.

 target/ppc/translate.c | 46 +++++++++++++++++++++++++-----------------
 1 file changed, 27 insertions(+), 19 deletions(-)

diff --git a/target/ppc/translate.c b/target/ppc/translate.c
index b207fb5386..9959259dba 100644
--- a/target/ppc/translate.c
+++ b/target/ppc/translate.c
@@ -4361,30 +4361,34 @@ static void gen_mtmsrd(DisasContext *ctx)
     CHK_SV;
 
 #if !defined(CONFIG_USER_ONLY)
+    if (tb_cflags(ctx->base.tb) & CF_USE_ICOUNT) {
+        gen_io_start();
+    }
     if (ctx->opcode & 0x00010000) {
-        /* Special form that does not need any synchronisation */
+        /* L=1 form only updates EE and RI */
         TCGv t0 = tcg_temp_new();
+        TCGv t1 = tcg_temp_new();
         tcg_gen_andi_tl(t0, cpu_gpr[rS(ctx->opcode)],
                         (1 << MSR_RI) | (1 << MSR_EE));
-        tcg_gen_andi_tl(cpu_msr, cpu_msr,
+        tcg_gen_andi_tl(t1, cpu_msr,
                         ~(target_ulong)((1 << MSR_RI) | (1 << MSR_EE)));
-        tcg_gen_or_tl(cpu_msr, cpu_msr, t0);
+        tcg_gen_or_tl(t1, t1, t0);
+
+        gen_helper_store_msr(cpu_env, t1);
         tcg_temp_free(t0);
+        tcg_temp_free(t1);
+
     } else {
         /*
          * XXX: we need to update nip before the store if we enter
          *      power saving mode, we will exit the loop directly from
          *      ppc_store_msr
          */
-        if (tb_cflags(ctx->base.tb) & CF_USE_ICOUNT) {
-            gen_io_start();
-        }
         gen_update_nip(ctx, ctx->base.pc_next);
         gen_helper_store_msr(cpu_env, cpu_gpr[rS(ctx->opcode)]);
-        /* Must stop the translation as machine state (may have) changed */
-        /* Note that mtmsr is not always defined as context-synchronizing */
-        gen_stop_exception(ctx);
     }
+    /* Must stop the translation as machine state (may have) changed */
+    gen_stop_exception(ctx);
 #endif /* !defined(CONFIG_USER_ONLY) */
 }
 #endif /* defined(TARGET_PPC64) */
@@ -4394,15 +4398,23 @@ static void gen_mtmsr(DisasContext *ctx)
     CHK_SV;
 
 #if !defined(CONFIG_USER_ONLY)
-   if (ctx->opcode & 0x00010000) {
-        /* Special form that does not need any synchronisation */
+    if (tb_cflags(ctx->base.tb) & CF_USE_ICOUNT) {
+        gen_io_start();
+    }
+    if (ctx->opcode & 0x00010000) {
+        /* L=1 form only updates EE and RI */
         TCGv t0 = tcg_temp_new();
+        TCGv t1 = tcg_temp_new();
         tcg_gen_andi_tl(t0, cpu_gpr[rS(ctx->opcode)],
                         (1 << MSR_RI) | (1 << MSR_EE));
-        tcg_gen_andi_tl(cpu_msr, cpu_msr,
+        tcg_gen_andi_tl(t1, cpu_msr,
                         ~(target_ulong)((1 << MSR_RI) | (1 << MSR_EE)));
-        tcg_gen_or_tl(cpu_msr, cpu_msr, t0);
+        tcg_gen_or_tl(t1, t1, t0);
+
+        gen_helper_store_msr(cpu_env, t1);
         tcg_temp_free(t0);
+        tcg_temp_free(t1);
+
     } else {
         TCGv msr = tcg_temp_new();
 
@@ -4411,9 +4423,6 @@ static void gen_mtmsr(DisasContext *ctx)
          *      power saving mode, we will exit the loop directly from
          *      ppc_store_msr
          */
-        if (tb_cflags(ctx->base.tb) & CF_USE_ICOUNT) {
-            gen_io_start();
-        }
         gen_update_nip(ctx, ctx->base.pc_next);
 #if defined(TARGET_PPC64)
         tcg_gen_deposit_tl(msr, cpu_msr, cpu_gpr[rS(ctx->opcode)], 0, 32);
@@ -4422,10 +4431,9 @@ static void gen_mtmsr(DisasContext *ctx)
 #endif
         gen_helper_store_msr(cpu_env, msr);
         tcg_temp_free(msr);
-        /* Must stop the translation as machine state (may have) changed */
-        /* Note that mtmsr is not always defined as context-synchronizing */
-        gen_stop_exception(ctx);
     }
+    /* Must stop the translation as machine state (may have) changed */
+    gen_stop_exception(ctx);
 #endif
 }
 
-- 
2.23.0