> On 10. Feb 2026, at 11:17, Peter Maydell <[email protected]> wrote:
>
> We log a GUEST_ERROR message "PL011 data written to disabled UART" if
> the guest writes data to the TX FIFO when it has not set the enable
> bit in the UART. The idea is to note that the guest has done
> something dubious but let it work anyway. However, since we print
> this message for every output character, it floods the logs when
> running a guest that does this.
>
> Keep a note of whether we've printed the log message or not, so we
> only output it once. If the guest actively disables the UART, we
> re-arm the log message.
>
> Notably, the Linux kernel does not bother to enable the UART if it is
> used for earlycon, relying on the firmware having already done that.
>
> Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Mohamed Mediouni <[email protected]>
> ---
> The log messages here were getting in my way trying to debug something
> else, but I didn't want to just delete them completely.
>
> hw/char/pl011.c | 24 ++++++++++++++++++++++--
> include/hw/char/pl011.h | 1 +
> 2 files changed, 23 insertions(+), 2 deletions(-)
>
> diff --git a/hw/char/pl011.c b/hw/char/pl011.c
> index b2ca68e3e8..cb12c3e224 100644
> --- a/hw/char/pl011.c
> +++ b/hw/char/pl011.c
> @@ -227,10 +227,25 @@ static void pl011_loopback_tx(PL011State *s, uint32_t
> value)
> static void pl011_write_txdata(PL011State *s, uint8_t data)
> {
> if (!(s->cr & CR_UARTEN)) {
> - qemu_log_mask(LOG_GUEST_ERROR,
> - "PL011 data written to disabled UART\n");
> + /*
> + * Only log this message once, not every time the guest outputs:
> + * otherwise we would flood the logs with this message, making
> + * harder to debug guests. (Some very popular guests like Linux
> + * don't actively enable the UART.)
> + */
> + if (!s->logged_disabled_uart) {
> + qemu_log_mask(LOG_GUEST_ERROR,
> + "PL011 data written to disabled UART\n");
> + s->logged_disabled_uart = true;
> + }
> }
> if (!(s->cr & CR_TXE)) {
> + /*
> + * We don't bother with the only-log-once machinery for this check
> + * because TXE is enabled by default from PL011 reset, so there
> + * isn't likely to be existing in-the-wild guest code that trips
> + * over this one.
> + */
> qemu_log_mask(LOG_GUEST_ERROR,
> "PL011 data written to disabled TX UART\n");
> }
> @@ -457,6 +472,10 @@ static void pl011_write(void *opaque, hwaddr offset,
> break;
> case 12: /* UARTCR */
> /* ??? Need to implement the enable bit. */
> + if ((s->cr ^ value) & CR_UARTEN) {
> + /* Re-arm the log warning when the guest toggles UARTEN */
> + s->logged_disabled_uart = false;
> + }
> s->cr = value;
> pl011_loopback_mdmctrl(s);
> break;
> @@ -665,6 +684,7 @@ static void pl011_reset(DeviceState *dev)
> s->ifl = 0x12;
> s->cr = 0x300;
> s->flags = 0;
> + s->logged_disabled_uart = false;
> pl011_reset_rx_fifo(s);
> pl011_reset_tx_fifo(s);
> }
> diff --git a/include/hw/char/pl011.h b/include/hw/char/pl011.h
> index ff735b5234..5695787650 100644
> --- a/include/hw/char/pl011.h
> +++ b/include/hw/char/pl011.h
> @@ -51,6 +51,7 @@ struct PL011State {
> qemu_irq irq[6];
> Clock *clk;
> bool migrate_clk;
> + bool logged_disabled_uart;
> const unsigned char *id;
> /*
> * Since some users embed this struct directly, we must
> --
> 2.43.0
>
>
