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Re: [PATCH for 6.1 1/2] ui/gtk: add a keyboard fifo to the VTE consoles

From: Volker Rümelin
Subject: Re: [PATCH for 6.1 1/2] ui/gtk: add a keyboard fifo to the VTE consoles
Date: Wed, 21 Jul 2021 23:11:56 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.12.0

+static void gd_vc_send_chars(VirtualConsole *vc)
+    uint32_t len, avail;
+    const uint8_t *buf;
+    len = qemu_chr_be_can_write(vc->vte.chr);
+    avail = fifo8_num_used(&vc->vte.out_fifo);
+    if (len > avail) {
+        len = avail;
+    }
+    while (len > 0) {
+        uint32_t size;
+        buf = fifo8_pop_buf(&vc->vte.out_fifo, len, &size);
+        qemu_chr_be_write(vc->vte.chr, (uint8_t *)buf, size);
+        len -= size;
+        avail -= size;
+    }
+    /*
+     * characters are pending: we send them a bit later (XXX:
+     * horrible, should change char device API)
+     */
+    if (avail > 0) {
+        timer_mod(vc->vte.kbd_timer,
+                  qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL) + 1);
+    }
There is ChardevClass->chr_accept_input() which gets called when you can
send more data, so there is no need to use a timer for that.

Oh, I didn't notice this callback function. With this, the retry timer and my attempt to quickly slow down the write retries are really not necessary.

Typical workflow is to only read data when it can be pushed forward to
the guest, so when the guest stops reading data qemu stops doing so too,
effectively forwarding the stalls.  Which works fine for things like tcp
sockets.  Not so much for user input though.

So, yes, just throw away data is the only option we have here.  Adding a
reasonable-sized fifo makes sense too to cover bulky input, so you can
cut+paste a longish URL even if the guest accepts only a few chars at a
time (16550 fifo is 16 chars IIRC ...).

I would suggest to keep things simple, just throw away what you can't
store in the fifo, I don't see the point taking different actions
depending on how long the stalls are lasting (patch 2/2).

I will send a version 2 patch.

With best regards,

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