This is responsible for failure of migration from 2.2 to 2.1, because thr_ipending is always one in practice. Calling serial_update_irq is the right thing to do indeed, because writing to IER could cause an interrupt to appear. However, there is no reason to set thr_ipending again.
This was already reported in 2010. See this quote from https://lists.gnu.org/archive/html/qemu-devel/2010-03/msg01914.html: > The commit in r1049 (serial interrupt fix (Hampa Hug)) prevents > booting Digital Research DOSPlus. Following patch partially reverts > that commit and makes DOSPlus booting in QEMU again. Bochs does not check LSR_THRE in IER, and the log message in r1049 doesn't explain why the change was made in the first place. This does not change the migration format, so 2.2.0 -> 2.1 will remain broken but we can fix 2.2.1 -> 2.1 without breaking 2.2.1 <-> 2.2.0. Signed-off-by: Paolo Bonzini <pbonz...@redhat.com> --- hw/char/serial.c | 5 +---- 1 file changed, 1 insertion(+), 4 deletions(-) diff --git a/hw/char/serial.c b/hw/char/serial.c index ebcacdc..cf8e4e3 100644 --- a/hw/char/serial.c +++ b/hw/char/serial.c @@ -350,10 +350,7 @@ static void serial_ioport_write(void *opaque, hwaddr addr, uint64_t val, s->poll_msl = 0; } } - if (s->lsr & UART_LSR_THRE) { - s->thr_ipending = 1; - serial_update_irq(s); - } + serial_update_irq(s); } break; case 2: -- 2.1.0
