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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 09:34:36 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20091209 Fedora/3.0-4.fc12 Lightning/1.0pre Thunderbird/3.0

On 05/04/2010 09:30 AM, Alexander Graf wrote:

Am 04.05.2010 um 15:44 schrieb Anthony Liguori <address@hidden>:

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.

There is no polling for the muxer. That's why it never knows when virtio-console can receive again.

Maybe I'm missing something simple, but it looks to me like the muxer is polling. mux_chr_can_read() is going to eventually poll the muxed devices to figure this out.

If the root of the problem is that mux_chr_can_read() isn't being invoked for a prolonged period of time, the real issue is the problem I described.


Anthony Liguori

This patch basically adfs timer based polling for that exact case.


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 *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;

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