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spice-qemu-char: Fix flow control in client -> guest direction

In the old spice-vmc device we used to have:
last_out = virtio_serial_write(&svc->port, p, MIN(len, VMC_MAX_HOST_WRITE));
if (last_out > 0)
   ...

Now in the chardev backend we have:
last_out = MIN(len, VMC_MAX_HOST_WRITE);
qemu_chr_read(scd->chr, p, last_out);
if (last_out > 0) {
   ...

Which causes us to no longer detect if the virtio port is not ready
to receive data from us. chardev actually has a mechanism to detect this,
but it requires a separate call to qemu_chr_can_read, before calling
qemu_chr_read (which return void).

This patch uses qemu_chr_can_read to fix the flow control from client to
guest.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
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commit 35106c2df2eda83e5f7fea356d80c11fed93df1f 1 parent d800040
@jwrdegoede jwrdegoede authored kraxel committed
Showing with 5 additions and 6 deletions.
  1. +5 −6 spice-qemu-char.c
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11 spice-qemu-char.c
@@ -36,14 +36,13 @@ static int vmc_write(SpiceCharDeviceInstance *sin, const uint8_t *buf, int len)
while (len > 0) {
last_out = MIN(len, VMC_MAX_HOST_WRITE);
- qemu_chr_read(scd->chr, p, last_out);
- if (last_out > 0) {
- out += last_out;
- len -= last_out;
- p += last_out;
- } else {
+ if (qemu_chr_can_read(scd->chr) < last_out) {
break;
}
+ qemu_chr_read(scd->chr, p, last_out);
+ out += last_out;
+ len -= last_out;
+ p += last_out;
}
dprintf(scd, 3, "%s: %lu/%zd\n", __func__, out, len + out);
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