diff options
authorPaolo Bonzini <pbonzini@redhat.com>2013-11-20 12:54:02 +0100
committerAnthony Liguori <aliguori@amazon.com>2013-11-21 07:55:45 -0800
commit81f3053b77f7d3a4d9100c425cd8cec99ee7a3d4 (patch)
parent1eb1bd9eafa890f1f4d16ef5cb8b9239a86874d9 (diff)
target-i386: yield to another VCPU on PAUSE
After commit b1bbfe7 (aio / timers: On timer modification, qemu_notify or aio_notify, 2013-08-21) FreeBSD guests report a huge slowdown. The problem shows up as soon as FreeBSD turns out its periodic (~1 ms) tick, but the timers are only the trigger for a pre-existing problem. Before the offending patch, setting a timer did a timer_settime system call. After, setting the timer exits the event loop (which uses poll) and reenters it with a new deadline. This does not cause any slowdown; the difference is between one system call (timer_settime and a signal delivery (SIGALRM) before the patch, and two system calls afterwards (write to a pipe or eventfd + calling poll again when re-entering the event loop). Unfortunately, the exit/enter causes the main loop to grab the iothread lock, which in turns kicks the VCPU thread out of execution. This causes TCG to execute the next VCPU in its round-robin scheduling of VCPUS. When the second VCPU is mostly unused, FreeBSD runs a "pause" instruction in its idle loop which only burns cycles without any progress. As soon as the timer tick expires, the first VCPU runs the interrupt handler but very soon it sets it again---and QEMU then goes back doing nothing in the second VCPU. The fix is to make the pause instruction do "cpu_loop_exit". Cc: Richard Henderson <rth@twiddle.net> Reported-by: Luigi Rizzo <rizzo@iet.unipi.it> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Richard Henderson <rth@twiddle.net> Message-id: 1384948442-24217-1-git-send-email-pbonzini@redhat.com Signed-off-by: Anthony Liguori <aliguori@amazon.com>
3 files changed, 25 insertions, 3 deletions
diff --git a/target-i386/helper.h b/target-i386/helper.h
index d6974dfd6b..3775abeba7 100644
--- a/target-i386/helper.h
+++ b/target-i386/helper.h
@@ -58,6 +58,7 @@ 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 93933fd162..b6307ca386 100644
--- a/target-i386/misc_helper.c
+++ b/target-i386/misc_helper.c
@@ -566,6 +566,15 @@ void helper_rdmsr(CPUX86State *env)
+static void do_pause(X86CPU *cpu)
+ CPUX86State *env = &cpu->env;
+ /* Just let another CPU run. */
+ env->exception_index = EXCP_INTERRUPT;
+ cpu_loop_exit(env);
static void do_hlt(X86CPU *cpu)
CPUState *cs = CPU(cpu);
@@ -611,13 +620,22 @@ 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) {
- /* more than one CPU: do not sleep because another CPU may
- wake this one */
+ do_pause(cpu);
} else {
+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)
env->exception_index = EXCP_DEBUG;
diff --git a/target-i386/translate.c b/target-i386/translate.c
index eb0ea93dbb..ecf16b389b 100644
--- a/target-i386/translate.c
+++ b/target-i386/translate.c
@@ -7224,7 +7224,10 @@ static target_ulong disas_insn(CPUX86State *env, DisasContext *s,
goto do_xchg_reg_eax;
if (prefixes & PREFIX_REPZ) {
- gen_svm_check_intercept(s, pc_start, SVM_EXIT_PAUSE);
+ 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;
case 0x9b: /* fwait */