target/i386: Give IRQs a chance when resetting HF_INHIBIT_IRQ_MASK

When emulated with QEMU, interrupts will never come in the following
loop. However, if the NOP instruction is uncommented, interrupts will
fire as normal.

	loop:
		cli
    		call do_sti
		jmp loop

	do_sti:
		sti
		# nop
		ret

This behavior is different from that of a real processor. For example,
if KVM is enabled, interrupts will always fire regardless of whether the
NOP instruction is commented or not. Also, the Intel Software Developer
Manual states that after the STI instruction is executed, the interrupt
inhibit should end as soon as the next instruction (e.g., the RET
instruction if the NOP instruction is commented) is executed.

This problem is caused because the previous code may choose not to end
the TB even if the HF_INHIBIT_IRQ_MASK has just been reset (e.g., in the
case where the STI instruction is immediately followed by the RET
instruction), so that IRQs may not have a change to trigger. This commit
fixes the problem by always terminating the current TB to give IRQs a
chance to trigger when HF_INHIBIT_IRQ_MASK is reset.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
Message-ID: <20240415064518.4951-4-lrh2000@pku.edu.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/target/i386/tcg/translate.c b/target/i386/tcg/translate.c
index c05d9e52259b8f5dfbd92fe92b9bce6a65bb8790..051ffb5e1fdcfc4cd0f3ef22bdc929fd3028b156 100644 (file)
--- a/target/i386/tcg/translate.c
+++ b/target/i386/tcg/translate.c
@@ -2798,13 +2798,17 @@ static void gen_bnd_jmp(DisasContext *s)
 static void
 do_gen_eob_worker(DisasContext *s, bool inhibit, bool recheck_tf, bool jr)
 {
+    bool inhibit_reset;
+
     gen_update_cc_op(s);
 
     /* If several instructions disable interrupts, only the first does it.  */
-    if (inhibit && !(s->flags & HF_INHIBIT_IRQ_MASK)) {
-        gen_set_hflag(s, HF_INHIBIT_IRQ_MASK);
-    } else {
+    inhibit_reset = false;
+    if (s->flags & HF_INHIBIT_IRQ_MASK) {
         gen_reset_hflag(s, HF_INHIBIT_IRQ_MASK);
+        inhibit_reset = true;
+    } else if (inhibit) {
+        gen_set_hflag(s, HF_INHIBIT_IRQ_MASK);
     }
 
     if (s->base.tb->flags & HF_RF_MASK) {
@@ -2815,7 +2819,9 @@ do_gen_eob_worker(DisasContext *s, bool inhibit, bool recheck_tf, bool jr)
         tcg_gen_exit_tb(NULL, 0);
     } else if (s->flags & HF_TF_MASK) {
         gen_helper_single_step(tcg_env);
-    } else if (jr) {
+    } else if (jr &&
+               /* give irqs a chance to happen */
+               !inhibit_reset) {
         tcg_gen_lookup_and_goto_ptr();
     } else {
         tcg_gen_exit_tb(NULL, 0);