When the frontend device has no space for a read the fd is removed from polling to allow time for the guest to read and clear the buffer. Without the call to qemu_chr_fe_accept_input(), the poll will not be broken out of when the guest has cleared the buffer causing significant IO delays that get worse with smaller buffers.
Buglink: https://bugs.launchpad.net/qemu/+bug/1913341 Signed-off-by: Iris Johnson <i...@modwiz.com> --- hw/char/exynos4210_uart.c | 1 + 1 file changed, 1 insertion(+) diff --git a/hw/char/exynos4210_uart.c b/hw/char/exynos4210_uart.c index 6361df2ad3..51ba862d0b 100644 --- a/hw/char/exynos4210_uart.c +++ b/hw/char/exynos4210_uart.c @@ -519,6 +519,7 @@ static uint64_t exynos4210_uart_read(void *opaque, hwaddr offset, s->reg[I_(UTRSTAT)] &= ~UTRSTAT_Rx_BUFFER_DATA_READY; res = s->reg[I_(URXH)]; } + qemu_chr_fe_accept_input(&s->chr); exynos4210_uart_update_dmabusy(s); trace_exynos_uart_read(s->channel, offset, exynos4210_uart_regname(offset), res); -- 2.25.1