kvm tools: Send SysRq-P to guest kernel on SIGQUIT

This patch makes SIGQUIT to the host hypervisor send 'SysRq-P' to the guest
kernel via 8250 serial console to make debugging stuck guests easier.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

8250-serial.c

@@ -53,6 +53,29 @@ static struct serial8250_device devices[] = {
 	},
 };
 
+#define SYSRQ_PENDING_NONE		0
+#define SYSRQ_PENDING_BREAK		1
+#define SYSRQ_PENDING_CMD		2
+
+static int sysrq_pending;
+
+static void serial8250__sysrq(struct kvm *self, struct serial8250_device *dev)
+{
+	switch (sysrq_pending) {
+	case SYSRQ_PENDING_BREAK:
+		dev->lsr	|= UART_LSR_DR | UART_LSR_BI;
+
+		sysrq_pending	= SYSRQ_PENDING_CMD;
+		break;
+	case SYSRQ_PENDING_CMD:
+		dev->rbr	= 'p';
+		dev->lsr	|= UART_LSR_DR;
+
+		sysrq_pending	= SYSRQ_PENDING_NONE;
+		break;
+	}
+}
+
 static void serial8250__receive(struct kvm *self, struct serial8250_device *dev)
 {
 	int c;
@@ -60,6 +83,11 @@ static void serial8250__receive(struct kvm *self, struct serial8250_device *dev)
 	if (dev->lsr & UART_LSR_DR)
 		return;
 
+	if (sysrq_pending) {
+		serial8250__sysrq(self, dev);
+		return;
+	}
+
 	if (!term_readable(CONSOLE_8250))
 		return;
 
@@ -94,6 +122,11 @@ void serial8250__inject_interrupt(struct kvm *self)
 	}
 }
 
+void serial8250__inject_sysrq(struct kvm *self)
+{
+	sysrq_pending	= SYSRQ_PENDING_BREAK;
+}
+
 static struct serial8250_device *find_device(uint16_t port)
 {
 	unsigned int i;

include/kvm/8250-serial.h

@@ -5,5 +5,6 @@ struct kvm;
 
 void serial8250__init(struct kvm *kvm);
 void serial8250__inject_interrupt(struct kvm *kvm);
+void serial8250__inject_sysrq(struct kvm *kvm);
 
 #endif /* KVM__8250_SERIAL_H */

main.c

@@ -38,6 +38,7 @@ static void handle_sigint(int sig)
 
 static void handle_sigquit(int sig)
 {
+	serial8250__inject_sysrq(kvm);
 	kvm__show_registers(kvm);
 	kvm__show_code(kvm);
 	kvm__show_page_tables(kvm);