D36979.diff
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	diff --git a/sys/dev/uart/uart_dev_ns8250.c b/sys/dev/uart/uart_dev_ns8250.c
	--- a/sys/dev/uart/uart_dev_ns8250.c
	+++ b/sys/dev/uart/uart_dev_ns8250.c
	@@ -210,6 +210,8 @@
	ns8250_flush(struct uart_bas *bas, int what)
	{
	uint8_t fcr;
	+ uint8_t lsr;
	+ int drain = 0;

	fcr = FCR_ENABLE;
	#ifdef CPU_XBURST
	@@ -221,6 +223,23 @@
	fcr \|= FCR_RCV_RST;
	uart_setreg(bas, REG_FCR, fcr);
	uart_barrier(bas);
	+
	+ /*
	+ * Detect and work around emulated UARTs which don't implement the
	+ * FCR register; on these systems we need to drain the FIFO since
	+ * the flush we request doesn't happen. One such system is the
	+ * Firecracker VMM, aka. the rust-vmm/vm-superio emulation code:
	+ * https://github.com/rust-vmm/vm-superio/issues/83
	+ */
	+ lsr = uart_getreg(bas, REG_LSR);
	+ if ((lsr & LSR_TEMT) && (what & UART_FLUSH_TRANSMITTER))
	+ drain \|= UART_DRAIN_TRANSMITTER;
	+ if ((lsr & LSR_RXRDY) && (what & UART_FLUSH_RECEIVER))
	+ drain \|= UART_DRAIN_RECEIVER;
	+ if (drain != 0) {
	+ printf("ns8250: UART FCR is broken\n");
	+ ns8250_drain(bas, drain);
	+ }
	}

	static int