[Qemu-devel] [PATCH 3/6] Make char muxer more robust wrt small FIFOs

[Alexander Graf <agraf@suse.de>]

Virtio-Console can only process one character at a time. Using it on S390
gave me strage "lags" where I got the character I pressed before when
pressing one. So I typed in "abc" and only received "a", then pressed "d"
but the guest received "b" and so on.

While the stdio driver calls a poll function that just processes on its
queue in case virtio-console can't take multiple characters at once, the
muxer does not have such callbacks, so it can't empty its queue.

To work around that limitation, I introduced a new timer that only gets
active when the guest can not receive any more characters. In that case
it polls again after a while to check if the guest is now receiving input.

This patch fixes input when using -nographic on s390 for me.

---

Please consider for stable.
---
 qemu-char.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

diff --git a/qemu-char.c b/qemu-char.c
index 048da3f..6ad6609 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -219,6 +219,7 @@ typedef struct {
     IOEventHandler *chr_event[MAX_MUX];
     void *ext_opaque[MAX_MUX];
     CharDriverState *drv;
+    QEMUTimer *accept_timer;
     int focus;
     int mux_cnt;
     int term_got_escape;
@@ -380,6 +381,13 @@ static void mux_chr_accept_input(CharDriverState *chr)
         d->chr_read[m](d->ext_opaque[m],
                        &d->buffer[m][d->cons[m]++ & MUX_BUFFER_MASK], 1);
     }
+
+    /* We're still not able to sync producer and consumer, so let's wait a bit
+       and try again by then. */
+    if (d->prod[m] != d->cons[m]) {
+        qemu_mod_timer(d->accept_timer, qemu_get_clock(vm_clock)
+                                        + (int64_t)100000);
+    }
 }
 
 static int mux_chr_can_read(void *opaque)
@@ -462,6 +470,8 @@ static CharDriverState *qemu_chr_open_mux(CharDriverState *drv)
     chr->opaque = d;
     d->drv = drv;
     d->focus = -1;
+    d->accept_timer = qemu_new_timer(vm_clock,
+                                     (QEMUTimerCB*)mux_chr_accept_input, chr);
     chr->chr_write = mux_chr_write;
     chr->chr_update_read_handler = mux_chr_update_read_handler;
     chr->chr_accept_input = mux_chr_accept_input;
-- 
1.6.0.2