Commit 8dfae8be authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Will Deacon
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kvm tools: serial: Make it work with non rt guests as well

Sasha reported, that a non RT guest reports "too much work for irq 4"
with the previous serial overhaul.

The reason is, that the new code allows unlimited tx transfers, which
triggers the sanity check in the 8250.c interrupt handler.

Limit the consecutive TX chars to 16 and let the guest kernel escape
from the 8250 interrupt handler. Set the TEMT/THRE bits in the
periodic serial console update.

Reported-by: default avatarSasha Levin <>
Signed-off-by: default avatarThomas Gleixner <>
Signed-off-by: default avatarPekka Enberg <>
parent f6b8ccc1
......@@ -19,6 +19,7 @@ struct serial8250_device {
u16 iobase;
u8 irq;
u8 irq_state;
int txcnt;
u8 rbr; /* receive buffer */
u8 dll;
......@@ -105,6 +106,16 @@ static void serial8250_update_irq(struct kvm *kvm, struct serial8250_device *dev
kvm__irq_line(kvm, dev->irq, 1);
dev->irq_state = iir;
* If the kernel disabled the tx interrupt, we know that there
* is nothing more to transmit, so we can reset our tx logic
* here.
if (!(dev->ier & UART_IER_THRI)) {
dev->txcnt = 0;
......@@ -134,6 +145,15 @@ static void serial8250__receive(struct kvm *kvm, struct serial8250_device *dev)
int c;
* If the guest transmitted 16 chars in a row, we clear the
* TEMT/THRE bits to let the kernel escape from the 8250
* interrupt handler. We come here only once a ms, so that
* should give the kernel the desired pause.
dev->txcnt = 0;
if (dev->lsr & UART_LSR_DR)
......@@ -212,14 +232,8 @@ static bool serial8250_out(struct ioport *ioport, struct kvm *kvm, u16 port, voi
term_putc(CONSOLE_8250, addr, size, dev->id);
/* else FIXME: Inject data into rcv path for LOOP */
* Set transmitter and transmit hold register
* empty. We have no FIFO at the moment and
* on the TX side it's only interesting, when
* we could coalesce port io on the kernel
* kernel.
if (++dev->txcnt == 16)
dev->lsr &= ~(UART_LSR_TEMT | UART_LSR_THRE);
} else {
dev->dll = ioport__read8(data);
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