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Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs
Date: Tue, 04 May 2010 08:44:09 -0500
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On 04/20/2010 11:56 AM, Alexander Graf wrote:
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.

I think this is really a kvm issue. I assume it's because s390 idles in the kernel so you never drop to userspace to repoll the descriptor.

A timer is a hacky solution. You really need to use an io thread to solve this and then you need to switch away from qemu_set_fd_handler2 to qemu_set_fd_handler() and make sure that the later breaks select whenever it's invoked.


Anthony Liguori


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 05df971..ce9df3a 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -235,6 +235,7 @@ 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;
@@ -396,6 +397,13 @@ static void mux_chr_accept_input(CharDriverState *chr)
                         &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)
@@ -478,6 +486,8 @@ static CharDriverState *qemu_chr_open_mux(CharDriverState 
      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;

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