[Qemu-devel] [RFC 38/38] Revert "target-i386: yield to another VCPU on PAUSE"

[Emilio G. Cota]

This reverts commit 81f3053b77f7d3a4d9100c425cd8cec99ee7a3d4.

The interrupt raised by the change in the commit above
kills performance when running many idling VCPUs. For example,
on my 64-core host when running a workload where cores are
idling often (e.g. blackscholes), performance drops significantly
because threads are most of the time just exiting the CPU loop,
thereby causing great contention on the BQL.

Fix it by reverting to the old behaviour by which no
interrupt is raised, which shouldn't be an issue given that
we have now one thread per VCPU.

Signed-off-by: Emilio G. Cota <address@hidden>

Conflicts:
        target-i386/misc_helper.c
---
 target-i386/helper.h      |  1 -
 target-i386/misc_helper.c | 22 ++--------------------
 target-i386/translate.c   |  5 +----
 3 files changed, 3 insertions(+), 25 deletions(-)

diff --git a/target-i386/helper.h b/target-i386/helper.h
index 7d92140..495d9f8 100644
--- a/target-i386/helper.h
+++ b/target-i386/helper.h
@@ -56,7 +56,6 @@ DEF_HELPER_2(sysret, void, env, int)
 DEF_HELPER_2(hlt, void, env, int)
 DEF_HELPER_2(monitor, void, env, tl)
 DEF_HELPER_2(mwait, void, env, int)
-DEF_HELPER_2(pause, void, env, int)
 DEF_HELPER_1(debug, void, env)
 DEF_HELPER_1(reset_rf, void, env)
 DEF_HELPER_3(raise_interrupt, void, env, int, int)
diff --git a/target-i386/misc_helper.c b/target-i386/misc_helper.c
index 52c5d65..0389df2 100644
--- a/target-i386/misc_helper.c
+++ b/target-i386/misc_helper.c
@@ -556,15 +556,6 @@ void helper_rdmsr(CPUX86State *env)
 }
 #endif
 
-static void do_pause(X86CPU *cpu)
-{
-    CPUState *cs = CPU(cpu);
-
-    /* Just let another CPU run.  */
-    cs->exception_index = EXCP_INTERRUPT;
-    cpu_loop_exit(cs);
-}
-
 static void do_hlt(X86CPU *cpu)
 {
     CPUState *cs = CPU(cpu);
@@ -610,22 +601,13 @@ void helper_mwait(CPUX86State *env, int next_eip_addend)
     cs = CPU(cpu);
     /* XXX: not complete but not completely erroneous */
     if (cs->cpu_index != 0 || CPU_NEXT(cs) != NULL) {
-        do_pause(cpu);
+        /* more than one CPU: do not sleep because another CPU may
+           wake this one */
     } else {
         do_hlt(cpu);
     }
 }
 
-void helper_pause(CPUX86State *env, int next_eip_addend)
-{
-    X86CPU *cpu = x86_env_get_cpu(env);
-
-    cpu_svm_check_intercept_param(env, SVM_EXIT_PAUSE, 0);
-    env->eip += next_eip_addend;
-
-    do_pause(cpu);
-}
-
 void helper_debug(CPUX86State *env)
 {
     CPUState *cs = CPU(x86_env_get_cpu(env));
diff --git a/target-i386/translate.c b/target-i386/translate.c
index 4d6030f..3b68660 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -6934,10 +6934,7 @@ static target_ulong disas_insn(CPUX86State *env, 
DisasContext *s,
             goto do_xchg_reg_eax;
         }
         if (prefixes & PREFIX_REPZ) {
-            gen_update_cc_op(s);
-            gen_jmp_im(pc_start - s->cs_base);
-            gen_helper_pause(cpu_env, tcg_const_i32(s->pc - pc_start));
-            s->is_jmp = DISAS_TB_JUMP;
+            gen_svm_check_intercept(s, pc_start, SVM_EXIT_PAUSE);
         }
         break;
     case 0x9b: /* fwait */
-- 
1.9.1