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[Xen-devel] Re: [RFC PATCH 29/35] Add the Xen virtual console driver.



Chris Wright wrote:
+/*
+ * Modes:
+ *  'xencons=off'  [XC_OFF]:     Console is disabled.
+ *  'xencons=tty'  [XC_TTY]:     Console attached to '/dev/tty[0-9]+'.
+ *  'xencons=ttyS' [XC_SERIAL]:  Console attached to '/dev/ttyS[0-9]+'.
+ *                 [XC_DEFAULT]: DOM0 -> XC_SERIAL ; all others -> XC_TTY.
+ * + * NB. In mode XC_TTY, we create dummy consoles for tty2-63. This suppresses
+ * warnings from standard distro startup scripts.
+ */

This has always seemed a bit wrong to me and makes a number of things kind of awkward (like a virtual video driver).

It would seem better me to treat this driver as what it really is, a virtual serial device. It adds a little bit of additional work to the userspace tools (they just have to make sure to pass console=ttyS0) but it seems worth it.

We could also solve the tty[0-9] problem by implementing a proper console driver that could use multiple virtual serial devices if we wanted to go that route.

Another option would be to just emulate a serial driver. The console driver isn't really performance critical. It seems to me that it's a bit unnecessary to even bother paravirtualizing the console device when it's so easy to emulate.

Regards,

Anthony Liguori

+static enum { XC_OFF, XC_DEFAULT, XC_TTY, XC_SERIAL } xc_mode = XC_DEFAULT;
+static int xc_num = -1;
+
+#ifdef CONFIG_MAGIC_SYSRQ
+static unsigned long sysrq_requested;
+extern int sysrq_enabled;
+#endif
+
+static int __init xencons_setup(char *str)
+{
+       char *q;
+       int n;
+
+       if (!strncmp(str, "ttyS", 4))
+               xc_mode = XC_SERIAL;
+       else if (!strncmp(str, "tty", 3))
+               xc_mode = XC_TTY;
+       else if (!strncmp(str, "off", 3))
+               xc_mode = XC_OFF;
+
+       switch (xc_mode) {
+       case XC_SERIAL:
+               n = simple_strtol(str+4, &q, 10);
+               if (q > (str + 4))
+                       xc_num = n;
+               break;
+       case XC_TTY:
+               n = simple_strtol(str+3, &q, 10);
+               if (q > (str + 3))
+                       xc_num = n;
+               break;
+       default:
+               break;
+       }
+
+       return 1;
+}
+__setup("xencons=", xencons_setup);
+
+/* The kernel and user-land drivers share a common transmit buffer. */
+static unsigned int wbuf_size = 4096;
+#define WBUF_MASK(_i) ((_i)&(wbuf_size-1))
+static char *wbuf;
+static unsigned int wc, wp; /* write_cons, write_prod */
+
+static int __init xencons_bufsz_setup(char *str)
+{
+       unsigned int goal;
+       goal = simple_strtoul(str, NULL, 0);
+       while (wbuf_size < goal)
+               wbuf_size <<= 1;
+       return 1;
+}
+__setup("xencons_bufsz=", xencons_bufsz_setup);
+
+/* This lock protects accesses to the common transmit buffer. */
+static spinlock_t xencons_lock = SPIN_LOCK_UNLOCKED;
+
+/* Common transmit-kick routine. */
+static void __xencons_tx_flush(void);
+
+static struct tty_driver *xencons_driver;
+
+/******************** Kernel console driver ********************************/
+
+static void kcons_write(
+       struct console *c, const char *s, unsigned int count)
+{
+       int           i = 0;
+       unsigned long flags;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+
+       while (i < count) {
+               for (; i < count; i++) {
+                       if ((wp - wc) >= (wbuf_size - 1))
+                               break;
+                       if ((wbuf[WBUF_MASK(wp++)] = s[i]) == '\n')
+                               wbuf[WBUF_MASK(wp++)] = '\r';
+               }
+
+               __xencons_tx_flush();
+       }
+
+       spin_unlock_irqrestore(&xencons_lock, flags);
+}
+
+static void kcons_write_dom0(
+       struct console *c, const char *s, unsigned int count)
+{
+       int rc;
+
+       while ((count > 0) &&
+              ((rc = HYPERVISOR_console_io(
+                       CONSOLEIO_write, count, (char *)s)) > 0)) {
+               count -= rc;
+               s += rc;
+       }
+}
+
+static struct tty_driver *kcons_device(struct console *c, int *index)
+{
+       *index = 0;
+       return xencons_driver;
+}
+
+static struct console kcons_info = {
+       .device = kcons_device,
+       .flags  = CON_PRINTBUFFER,
+       .index  = -1,
+};
+
+#define __RETCODE 0
+static int __init xen_console_init(void)
+{
+       if (xen_init() < 0)
+               return __RETCODE;
+
+       if (xen_start_info->flags & SIF_INITDOMAIN) {
+               if (xc_mode == XC_DEFAULT)
+                       xc_mode = XC_SERIAL;
+               kcons_info.write = kcons_write_dom0;
+               if (xc_mode == XC_SERIAL)
+                       kcons_info.flags |= CON_ENABLED;
+       } else {
+               if (xc_mode == XC_DEFAULT)
+                       xc_mode = XC_TTY;
+               kcons_info.write = kcons_write;
+       }
+
+       switch (xc_mode) {
+       case XC_SERIAL:
+               strcpy(kcons_info.name, "ttyS");
+               if (xc_num == -1)
+                       xc_num = 0;
+               break;
+
+       case XC_TTY:
+               strcpy(kcons_info.name, "tty");
+               if (xc_num == -1)
+                       xc_num = 1;
+               break;
+
+       default:
+               return __RETCODE;
+       }
+
+       wbuf = alloc_bootmem(wbuf_size);
+
+       register_console(&kcons_info);
+
+       return __RETCODE;
+}
+console_initcall(xen_console_init);
+
+/*** Useful function for console debugging -- goes straight to Xen. ***/
+asmlinkage int xprintk(const char *fmt, ...)
+{
+       va_list args;
+       int printk_len;
+       static char printk_buf[1024];
+
+       /* Emit the output into the temporary buffer */
+       va_start(args, fmt);
+       printk_len = vsnprintf(printk_buf, sizeof(printk_buf), fmt, args);
+       va_end(args);
+
+       /* Send the processed output directly to Xen. */
+       kcons_write_dom0(NULL, printk_buf, printk_len);
+
+       return 0;
+}
+
+/*** Forcibly flush console data before dying. ***/
+void xencons_force_flush(void)
+{
+       int sz;
+
+       /* Emergency console is synchronous, so there's nothing to flush. */
+       if (xen_start_info->flags & SIF_INITDOMAIN)
+               return;
+
+       /* Spin until console data is flushed through to the daemon. */
+       while (wc != wp) {
+               int sent = 0;
+               if ((sz = wp - wc) == 0)
+                       continue;
+               sent = xencons_ring_send(&wbuf[WBUF_MASK(wc)], sz);
+               if (sent > 0)
+                       wc += sent;
+       }
+}
+
+
+/******************** User-space console driver (/dev/console) ************/
+
+#define DRV(_d)         (_d)
+#define TTY_INDEX(_tty) ((_tty)->index)
+
+static struct termios *xencons_termios[MAX_NR_CONSOLES];
+static struct termios *xencons_termios_locked[MAX_NR_CONSOLES];
+static struct tty_struct *xencons_tty;
+static int xencons_priv_irq;
+static char x_char;
+
+void xencons_rx(char *buf, unsigned len, struct pt_regs *regs)
+{
+       int           i;
+       unsigned long flags;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+       if (xencons_tty == NULL)
+               goto out;
+
+       for (i = 0; i < len; i++) {
+#ifdef CONFIG_MAGIC_SYSRQ
+               if (sysrq_enabled) {
+                       if (buf[i] == '\x0f') { /* ^O */
+                               sysrq_requested = jiffies;
+                               continue; /* don't print the sysrq key */
+                       } else if (sysrq_requested) {
+                               unsigned long sysrq_timeout =
+                                       sysrq_requested + HZ*2;
+                               sysrq_requested = 0;
+                               if (time_before(jiffies, sysrq_timeout)) {
+                                       spin_unlock_irqrestore(
+                                               &xencons_lock, flags);
+                                       handle_sysrq(
+                                               buf[i], regs, xencons_tty);
+                                       spin_lock_irqsave(
+                                               &xencons_lock, flags);
+                                       continue;
+                               }
+                       }
+               }
+#endif
+               tty_insert_flip_char(xencons_tty, buf[i], 0);
+       }
+       tty_flip_buffer_push(xencons_tty);
+
+ out:
+       spin_unlock_irqrestore(&xencons_lock, flags);
+}
+
+static void __xencons_tx_flush(void)
+{
+       int sent, sz, work_done = 0;
+
+       if (x_char) {
+               if (xen_start_info->flags & SIF_INITDOMAIN)
+                       kcons_write_dom0(NULL, &x_char, 1);
+               else
+                       while (x_char)
+                               if (xencons_ring_send(&x_char, 1) == 1)
+                                       break;
+               x_char = 0;
+               work_done = 1;
+       }
+
+       while (wc != wp) {
+               sz = wp - wc;
+               if (sz > (wbuf_size - WBUF_MASK(wc)))
+                       sz = wbuf_size - WBUF_MASK(wc);
+               if (xen_start_info->flags & SIF_INITDOMAIN) {
+                       kcons_write_dom0(NULL, &wbuf[WBUF_MASK(wc)], sz);
+                       wc += sz;
+               } else {
+                       sent = xencons_ring_send(&wbuf[WBUF_MASK(wc)], sz);
+                       if (sent == 0)
+                               break;
+                       wc += sent;
+               }
+               work_done = 1;
+       }
+
+       if (work_done && (xencons_tty != NULL)) {
+               wake_up_interruptible(&xencons_tty->write_wait);
+               if ((xencons_tty->flags & (1 << TTY_DO_WRITE_WAKEUP)) &&
+                   (xencons_tty->ldisc.write_wakeup != NULL))
+                       (xencons_tty->ldisc.write_wakeup)(xencons_tty);
+       }
+}
+
+void xencons_tx(void)
+{
+       unsigned long flags;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+       __xencons_tx_flush();
+       spin_unlock_irqrestore(&xencons_lock, flags);
+}
+
+/* Privileged receive callback and transmit kicker. */
+static irqreturn_t xencons_priv_interrupt(int irq, void *dev_id,
+                                          struct pt_regs *regs)
+{
+       static char rbuf[16];
+       int         l;
+
+       while ((l = HYPERVISOR_console_io(CONSOLEIO_read, 16, rbuf)) > 0)
+               xencons_rx(rbuf, l, regs);
+
+       xencons_tx();
+
+       return IRQ_HANDLED;
+}
+
+static int xencons_write_room(struct tty_struct *tty)
+{
+       return wbuf_size - (wp - wc);
+}
+
+static int xencons_chars_in_buffer(struct tty_struct *tty)
+{
+       return wp - wc;
+}
+
+static void xencons_send_xchar(struct tty_struct *tty, char ch)
+{
+       unsigned long flags;
+
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+       x_char = ch;
+       __xencons_tx_flush();
+       spin_unlock_irqrestore(&xencons_lock, flags);
+}
+
+static void xencons_throttle(struct tty_struct *tty)
+{
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       if (I_IXOFF(tty))
+               xencons_send_xchar(tty, STOP_CHAR(tty));
+}
+
+static void xencons_unthrottle(struct tty_struct *tty)
+{
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       if (I_IXOFF(tty)) {
+               if (x_char != 0)
+                       x_char = 0;
+               else
+                       xencons_send_xchar(tty, START_CHAR(tty));
+       }
+}
+
+static void xencons_flush_buffer(struct tty_struct *tty)
+{
+       unsigned long flags;
+
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+       wc = wp = 0;
+       spin_unlock_irqrestore(&xencons_lock, flags);
+}
+
+static inline int __xencons_put_char(int ch)
+{
+       char _ch = (char)ch;
+       if ((wp - wc) == wbuf_size)
+               return 0;
+       wbuf[WBUF_MASK(wp++)] = _ch;
+       return 1;
+}
+
+static int xencons_write(
+       struct tty_struct *tty,
+       const unsigned char *buf,
+       int count)
+{
+       int i;
+       unsigned long flags;
+
+       if (TTY_INDEX(tty) != 0)
+               return count;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+
+       for (i = 0; i < count; i++)
+               if (!__xencons_put_char(buf[i]))
+                       break;
+
+       if (i != 0)
+               __xencons_tx_flush();
+
+       spin_unlock_irqrestore(&xencons_lock, flags);
+
+       return i;
+}
+
+static void xencons_put_char(struct tty_struct *tty, u_char ch)
+{
+       unsigned long flags;
+
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+       (void)__xencons_put_char(ch);
+       spin_unlock_irqrestore(&xencons_lock, flags);
+}
+
+static void xencons_flush_chars(struct tty_struct *tty)
+{
+       unsigned long flags;
+
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+       __xencons_tx_flush();
+       spin_unlock_irqrestore(&xencons_lock, flags);
+}
+
+static void xencons_wait_until_sent(struct tty_struct *tty, int timeout)
+{
+       unsigned long orig_jiffies = jiffies;
+
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       while (DRV(tty->driver)->chars_in_buffer(tty)) {
+               set_current_state(TASK_INTERRUPTIBLE);
+               schedule_timeout(1);
+               if (signal_pending(current))
+                       break;
+               if (timeout && time_after(jiffies, orig_jiffies + timeout))
+                       break;
+       }
+
+       set_current_state(TASK_RUNNING);
+}
+
+static int xencons_open(struct tty_struct *tty, struct file *filp)
+{
+       unsigned long flags;
+
+       if (TTY_INDEX(tty) != 0)
+               return 0;
+
+       spin_lock_irqsave(&xencons_lock, flags);
+       tty->driver_data = NULL;
+       if (xencons_tty == NULL)
+               xencons_tty = tty;
+       __xencons_tx_flush();
+       spin_unlock_irqrestore(&xencons_lock, flags);
+
+       return 0;
+}
+
+static void xencons_close(struct tty_struct *tty, struct file *filp)
+{
+       unsigned long flags;
+
+       if (TTY_INDEX(tty) != 0)
+               return;
+
+       if (tty->count == 1) {
+               tty->closing = 1;
+               tty_wait_until_sent(tty, 0);
+               if (DRV(tty->driver)->flush_buffer != NULL)
+                       DRV(tty->driver)->flush_buffer(tty);
+               if (tty->ldisc.flush_buffer != NULL)
+                       tty->ldisc.flush_buffer(tty);
+               tty->closing = 0;
+               spin_lock_irqsave(&xencons_lock, flags);
+               xencons_tty = NULL;
+               spin_unlock_irqrestore(&xencons_lock, flags);
+       }
+}
+
+static struct tty_operations xencons_ops = {
+       .open = xencons_open,
+       .close = xencons_close,
+       .write = xencons_write,
+       .write_room = xencons_write_room,
+       .put_char = xencons_put_char,
+       .flush_chars = xencons_flush_chars,
+       .chars_in_buffer = xencons_chars_in_buffer,
+       .send_xchar = xencons_send_xchar,
+       .flush_buffer = xencons_flush_buffer,
+       .throttle = xencons_throttle,
+       .unthrottle = xencons_unthrottle,
+       .wait_until_sent = xencons_wait_until_sent,
+};
+
+static int __init xencons_init(void)
+{
+       int rc;
+
+       if (xen_init() < 0)
+               return -ENODEV;
+
+       if (xc_mode == XC_OFF)
+               return 0;
+
+       xencons_ring_init();
+
+       xencons_driver = alloc_tty_driver((xc_mode == XC_SERIAL) ?
+                                         1 : MAX_NR_CONSOLES);
+       if (xencons_driver == NULL)
+               return -ENOMEM;
+
+       DRV(xencons_driver)->name            = "xencons";
+       DRV(xencons_driver)->major           = TTY_MAJOR;
+       DRV(xencons_driver)->type            = TTY_DRIVER_TYPE_SERIAL;
+       DRV(xencons_driver)->subtype         = SERIAL_TYPE_NORMAL;
+       DRV(xencons_driver)->init_termios    = tty_std_termios;
+       DRV(xencons_driver)->flags           =
+               TTY_DRIVER_REAL_RAW |
+               TTY_DRIVER_RESET_TERMIOS |
+               TTY_DRIVER_NO_DEVFS;
+       DRV(xencons_driver)->termios         = xencons_termios;
+       DRV(xencons_driver)->termios_locked  = xencons_termios_locked;
+
+       if (xc_mode == XC_SERIAL) {
+               DRV(xencons_driver)->name        = "ttyS";
+               DRV(xencons_driver)->minor_start = 64 + xc_num;
+               DRV(xencons_driver)->name_base   = 0 + xc_num;
+       } else {
+               DRV(xencons_driver)->name        = "tty";
+               DRV(xencons_driver)->minor_start = xc_num;
+               DRV(xencons_driver)->name_base   = xc_num;
+       }
+
+       tty_set_operations(xencons_driver, &xencons_ops);
+
+       if ((rc = tty_register_driver(DRV(xencons_driver))) != 0) {
+               printk("WARNING: Failed to register Xen virtual "
+                      "console driver as '%s%d'\n",
+                      DRV(xencons_driver)->name,
+                      DRV(xencons_driver)->name_base);
+               put_tty_driver(xencons_driver);
+               xencons_driver = NULL;
+               return rc;
+       }
+
+       tty_register_device(xencons_driver, 0, NULL);
+
+       if (xen_start_info->flags & SIF_INITDOMAIN) {
+               xencons_priv_irq = bind_virq_to_irqhandler(
+                       VIRQ_CONSOLE,
+                       0,
+                       xencons_priv_interrupt,
+                       0,
+                       "console",
+                       NULL);
+               BUG_ON(xencons_priv_irq < 0);
+       }
+
+       printk("Xen virtual console successfully installed as %s%d\n",
+              DRV(xencons_driver)->name,
+              DRV(xencons_driver)->name_base );
+
+       return 0;
+}
+
+module_init(xencons_init);
--- /dev/null
+++ xen-subarch-2.6/drivers/xen/console/xencons_ring.c
@@ -0,0 +1,115 @@
+#include <linux/version.h>
+#include <linux/module.h>
+#include <linux/errno.h>
+#include <linux/signal.h>
+#include <linux/sched.h>
+#include <linux/interrupt.h>
+#include <linux/tty.h>
+#include <linux/tty_flip.h>
+#include <linux/serial.h>
+#include <linux/major.h>
+#include <linux/ptrace.h>
+#include <linux/ioport.h>
+#include <linux/mm.h>
+#include <linux/slab.h>
+
+#include <asm/hypervisor.h>
+#include <xen/evtchn.h>
+#include <xen/xencons.h>
+#include <linux/wait.h>
+#include <linux/interrupt.h>
+#include <linux/sched.h>
+#include <linux/err.h>
+#include <xen/interface/io/console.h>
+
+static int xencons_irq;
+
+static inline struct xencons_interface *xencons_interface(void)
+{
+       return mfn_to_virt(xen_start_info->console_mfn);
+}
+
+static inline void notify_daemon(void)
+{
+       /* Use evtchn: this is called early, before irq is set up. */
+       notify_remote_via_evtchn(xen_start_info->console_evtchn);
+}
+
+int xencons_ring_send(const char *data, unsigned len)
+{
+       int sent = 0;
+       struct xencons_interface *intf = xencons_interface();
+       XENCONS_RING_IDX cons, prod;
+
+       cons = intf->out_cons;
+       prod = intf->out_prod;
+       mb();
+       BUG_ON((prod - cons) > sizeof(intf->out));
+
+       while ((sent < len) && ((prod - cons) < sizeof(intf->out)))
+               intf->out[MASK_XENCONS_IDX(prod++, intf->out)] = data[sent++];
+
+       wmb();
+       intf->out_prod = prod;
+
+       notify_daemon();
+
+       return sent;
+}
+
+static irqreturn_t handle_input(int irq, void *unused, struct pt_regs *regs)
+{
+       struct xencons_interface *intf = xencons_interface();
+       XENCONS_RING_IDX cons, prod;
+
+       cons = intf->in_cons;
+       prod = intf->in_prod;
+       mb();
+       BUG_ON((prod - cons) > sizeof(intf->in));
+
+       while (cons != prod) {
+               xencons_rx(intf->in+MASK_XENCONS_IDX(cons,intf->in), 1, regs);
+               cons++;
+       }
+
+       mb();
+       intf->in_cons = cons;
+
+       notify_daemon();
+
+       xencons_tx();
+
+       return IRQ_HANDLED;
+}
+
+int xencons_ring_init(void)
+{
+       int err;
+
+       if (xencons_irq)
+               unbind_from_irqhandler(xencons_irq, NULL);
+       xencons_irq = 0;
+
+       if (!xen_start_info->console_evtchn)
+               return 0;
+
+       err = bind_evtchn_to_irqhandler(
+               xen_start_info->console_evtchn,
+               handle_input, 0, "xencons", NULL);
+       if (err <= 0) {
+               printk(KERN_ERR "XEN console request irq failed %i\n", err);
+               return err;
+       }
+
+       xencons_irq = err;
+
+       /* In case we have in-flight data after save/restore... */
+       notify_daemon();
+
+       return 0;
+}
+
+void xencons_resume(void)
+{
+       (void)xencons_ring_init();
+}
--- /dev/null
+++ xen-subarch-2.6/include/xen/xencons.h
@@ -0,0 +1,14 @@
+#ifndef __ASM_XENCONS_H__
+#define __ASM_XENCONS_H__
+
+void xencons_force_flush(void);
+void xencons_resume(void);
+
+/* Interrupt work hooks. Receive data, or kick data out. */
+void xencons_rx(char *buf, unsigned len, struct pt_regs *regs);
+void xencons_tx(void);
+
+int xencons_ring_init(void);
+int xencons_ring_send(const char *data, unsigned len);
+
+#endif /* __ASM_XENCONS_H__ */

--
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