[Qemu-devel] Re: MIPS, io-thread, icount and wfi

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Mon, Jan 17, 2011 at 11:03:08AM +0100, Edgar E. Iglesias wrote:
> Hi,
> 
> I'm running an io-thread enabled qemu-system-mipsel with icount.
> When the guest (linux) goes to sleep through the wait insn (waiting
> to be woken up by future timer interrupts), the thing deadlocks.
> 
> IIUC, this is because vm timers are driven by icount, but the CPU is
> halted so icount makes no progress and time stands still.
> 
> I've locally disabled vcpu halting when icount is enabled, that
> works around my problem but of course makes qemu consume 100% host cpu.
> 
> I don't know why I only see this problem with io-thread builds?
> Could be related timing and luck.
> 
> Would be interesting to know if someone has any info on how this was
> intended to work (if it was)? And if there are ideas for better
> workarounds or fixes that don't disable vcpu halting entirely.

Hi,

I've found the problem. For some reason io-thread builds use a
static timeout for wait loops. The entire chunk of code that
makes sure qemu_icount makes forward progress when the CPU's
are idle has been ifdef'ed away...

This fixes the problem for me, hopefully without affecting
io-thread runs without icount.

commit 0f4f3a919952500b487b438c5520f07a1c6be35b
Author: Edgar E. Iglesias <edgar@axis.com>
Date:   Tue Jan 18 01:01:57 2011 +0100

    qemu-timer: Fix timeout calc for io-thread with icount
    
    Make sure we always make forward progress with qemu_icount to
    avoid deadlocks. For io-thread, use the static 1000 timeout
    only if icount is disabled.
    
    Signed-off-by: Edgar E. Iglesias <edgar@axis.com>

diff --git a/qemu-timer.c b/qemu-timer.c
index 95814af..db1ec49 100644
--- a/qemu-timer.c
+++ b/qemu-timer.c
@@ -110,7 +110,6 @@ static int64_t cpu_get_clock(void)
     }
 }
 
-#ifndef CONFIG_IOTHREAD
 static int64_t qemu_icount_delta(void)
 {
     if (!use_icount) {
@@ -124,7 +123,6 @@ static int64_t qemu_icount_delta(void)
         return cpu_get_icount() - cpu_get_clock();
     }
 }
-#endif
 
 /* enable cpu_get_ticks() */
 void cpu_enable_ticks(void)
@@ -1077,9 +1075,17 @@ void quit_timers(void)
 
 int qemu_calculate_timeout(void)
 {
-#ifndef CONFIG_IOTHREAD
     int timeout;
 
+#ifdef CONFIG_IOTHREAD
+    /* When using icount, making forward progress with qemu_icount when the
+       guest CPU is idle is critical. We only use the static io-thread timeout
+       for non icount runs.  */
+    if (!use_icount) {
+        return 1000;
+    }
+#endif
+
     if (!vm_running)
         timeout = 5000;
     else {
@@ -1110,8 +1116,5 @@ int qemu_calculate_timeout(void)
     }
 
     return timeout;
-#else /* CONFIG_IOTHREAD */
-    return 1000;
-#endif
 }