linux-kernel - Serial of guest kernel complain "too much work" in kvm/qemu

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Date:	Thu, 21 Aug 2008 14:03:56 +0800
From:	"Liu Yu" <Yu.Liu@...escale.com>
To:	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
	<kvm-ppc@...r.kernel.org>, <qemu-devel@...gnu.org>
Cc:	"Hollis Blanchard" <hollisb@...ibm.com>
Subject: Serial of guest kernel complain "too much work" in kvm/qemu

Hi all,

How is this going?
http://www.archivum.info/qemu-devel@nongnu.org/2008-02/msg00169.html

I use serial as guest's default stdio on powerpc platform.
When displaying a number of characters, e.g. cat a big ascii file,
the serial always hangs as it encounters overwhelming interrupts,
and then complains "serial8250: too much work for irq 42".

The problem is that:
serial in qemu generate an interrupt on every output character,
while Linux driver reads serial data in a loop but gives up after a fixed number (256) of iterations.
So if serial output more than 256 characters in a short time, the serial die.

I had to use the patch below to get it worked around.

So what is the best way to solve it?

---
diff --git a/drivers/serial/8250.c b/drivers/serial/8250.c index be95e55..af3e569 100644
--- a/drivers/serial/8250.c
+++ b/drivers/serial/8250.c
@@ -70,7 +70,7 @@ static unsigned int nr_uarts = CONFIG_SERIAL_8250_RUNTIME_UARTS;
 #define DEBUG_INTR(fmt...)	do { } while (0)
 #endif

-#define PASS_LIMIT	256
+#define PASS_LIMIT	65536

 /*
  * We default to IRQ0 for the "no irq" hack.   Some