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[Qemu-devel] Re: [PATCH] [RESEND] Make char muxer more robust wrt small
From: |
Jan Kiszka |
Subject: |
[Qemu-devel] Re: [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs |
Date: |
Wed, 05 May 2010 09:33:40 +0200 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv:1.8.1.12) Gecko/20080226 SUSE/2.0.0.12-1.1 Thunderbird/2.0.0.12 Mnenhy/0.7.5.666 |
Jan Kiszka wrote:
> Anthony Liguori wrote:
>> On 05/04/2010 11:01 AM, Alexander Graf wrote:
>>> Am 04.05.2010 um 16:34 schrieb Anthony Liguori <address@hidden>:
>>>
>>>> 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.
>>> The problem is that the select list of fds includes the stdio fd, so
>>> that gets notified and is coupled with virtio-console, but there's
>>> nothing passing that on to mux and I don't think it'd be clever to
>>> expose internal data to the muxer to tell it about the backend fds.
>> When stdio is readable, it should invoke qemu_chr_read() with the read
>> data which in turn ought to invoke mux_chr_read().
>>
>> I'm not sure I understand what signalling is missing. Jan, does the
>> problem Alex describes ring a bell? I seem to recall you saying that
>> mux was still fundamentally broken but ought to work most of the time...
>
> That problem was (and still is) that the muxer needs to accept
> characters even if the active front-end device is not in order to filter
> out control sequences. Once its queue is full, it will start dropping
> those the active device would not if directly connected. Could only be
> solved via some peek service on pending front-end data.
>
> I think Alex' problem can be addressed by registering
> qemu_set_fd_handler2(..., backend->read_poll, mux_chr_read, ...). That
> means the backend has to tell us about its read poll handler (if any).
Nonsense.
In fact, the problem is the former issue: As the muxer reads the
character the front-end is currently unable to receive, polling may stop
until as the back-end has some further chars to deliver.
But interestingly, the stdio back-end has a (single-byte) fifo as well.
It just drives it a bit differently.
Alex, does this help as well?
diff --git a/qemu-char.c b/qemu-char.c
index ac65a1c..2b115a4 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -404,6 +404,8 @@ static int mux_chr_can_read(void *opaque)
MuxDriver *d = chr->opaque;
int m = d->focus;
+ mux_chr_accept_input(opaque);
+
if ((d->prod[m] - d->cons[m]) < MUX_BUFFER_SIZE)
return 1;
if (d->chr_can_read[m])
@@ -418,8 +420,6 @@ static void mux_chr_read(void *opaque, const uint8_t *buf,
int size)
int m = d->focus;
int i;
- mux_chr_accept_input (opaque);
-
for(i = 0; i < size; i++)
if (mux_proc_byte(chr, d, buf[i])) {
if (d->prod[m] == d->cons[m] &&
I'm trying to reproduce in parallel.
Jan
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- Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Anthony Liguori, 2010/05/04
- Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Alexander Graf, 2010/05/04
- Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Anthony Liguori, 2010/05/04
- Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Alexander Graf, 2010/05/04
- Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Anthony Liguori, 2010/05/04
- Re: [Qemu-devel] [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Jan Kiszka, 2010/05/04
- [Qemu-devel] Re: [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs,
Jan Kiszka <=
- [Qemu-devel] Re: [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Jan Kiszka, 2010/05/05
- [Qemu-devel] Re: [PATCH] [RESEND] Make char muxer more robust wrt small FIFOs, Alexander Graf, 2010/05/05
- [Qemu-devel] [PATCH] char: Flush read buffer in mux_chr_can_read, Jan Kiszka, 2010/05/05
- [Qemu-devel] Re: [PATCH] char: Flush read buffer in mux_chr_can_read, Alexander Graf, 2010/05/11
- [Qemu-devel] Re: [PATCH] char: Flush read buffer in mux_chr_can_read, Jan Kiszka, 2010/05/11
- [Qemu-devel] Re: [PATCH] char: Flush read buffer in mux_chr_can_read, Alexander Graf, 2010/05/11
- [Qemu-devel] Re: [PATCH] char: Flush read buffer in mux_chr_can_read, Jan Kiszka, 2010/05/12
- [Qemu-devel] Re: [PATCH] char: Flush read buffer in mux_chr_can_read, Alexander Graf, 2010/05/14
- [Qemu-devel] Re: [PATCH] char: Flush read buffer in mux_chr_can_read, Jan Kiszka, 2010/05/14
- [Qemu-devel] Re: [PATCH] char: Flush read buffer in mux_chr_can_read, Alexander Graf, 2010/05/15