Re: [Xen-devel] [PATCH] xen/console: introduce a 'w' debug-key that dumps the console ring

[Matt Wilson <msw@xxxxxxxxxx>]

On Sat, Sep 29, 2012 at 06:55:41AM +0100, Keir Fraser wrote:
> On 29/09/2012 06:02, "Matt Wilson" <msw@xxxxxxxxxx> wrote:
> 
> > This patch adds a new 'w' debug-key, chosen from the limited remaining
> > keys only due to its proximity to 'q', that dumps the console ring to
> > configured console devices. It's useful to for tracking down how an
> > unresponsive system got into a broken state via serial console.
> 
> Why wouldn't everything on the ring already have been sent to the configured
> console devices?

It would have been. But what if no one was looking, or dom0 cleared
the console, or placed it into a scrolling mode that prevents a
virtual terminal from placing output into a scrollback buffer. With
this patch, you can connect to a serial console after a machine
becomes unresponsive and gather clues on what went wrong.

Matt

> > Signed-off-by: Matt Wilson <msw@xxxxxxxxxx>
> > 
> > diff -r bd953fda6106 -r 20f6976e28a1 xen/drivers/char/console.c
> > --- a/xen/drivers/char/console.c Fri Sep 28 10:59:41 2012 +0200
> > +++ b/xen/drivers/char/console.c Sat Sep 29 05:00:05 2012 +0000
> > @@ -264,6 +264,49 @@ static void sercon_puts(const char *s)
> >          serial_puts(sercon_handle, s);
> >  }
> >  
> > +static void dump_console_ring_key(unsigned char key)
> > +{
> > +    uint32_t idx, len, sofar, c;
> > +    unsigned int order;
> > +    char *buf;
> > +
> > +    printk("'%c' pressed -> dumping console ring buffer (dmesg)\n", key);
> > +
> > +    /* create a buffer in which we'll copy the ring in the correct
> > +       order and NUL terminate */
> > +    order = get_order_from_bytes(conring_size + 1);
> > +    buf = alloc_xenheap_pages(order, 0);
> > +    if ( buf == NULL )
> > +    {
> > +        printk("unable to allocate memory!\n");
> > +        return;
> > +    }
> > +
> > +    c = conringc;
> > +    sofar = 0;
> > +    while ( (c != conringp) )
> > +    {
> > +        idx = CONRING_IDX_MASK(c);
> > +        len = conringp - c;
> > +        if ( (idx + len) > conring_size )
> > +            len = conring_size - idx;
> > +        memcpy(buf + sofar, &conring[idx], len);
> > +        sofar += len;
> > +        c += len;
> > +    }
> > +    buf[sofar] = '\0';
> > +
> > +    sercon_puts(buf);
> > +    vga_puts(buf);
> > +
> > +    free_xenheap_pages(buf, order);
> > +}
> > +
> > +static struct keyhandler dump_console_ring_keyhandler = {
> > +    .u.fn = dump_console_ring_key,
> > +    .desc = "synchronously dump console ring buffer (dmesg)"
> > +};
> > +
> >  /* CTRL-<switch_char> switches input direction between Xen and DOM0. */
> >  #define switch_code (opt_conswitch[0]-'a'+1)
> >  static int __read_mostly xen_rx = 1; /* FALSE => serial input passed to
> > domain 0. */
> > @@ -661,6 +704,8 @@ void __init console_endboot(void)
> >      if ( opt_conswitch[1] == 'x' )
> >          xen_rx = !xen_rx;
> >  
> > +    register_keyhandler('w', &dump_console_ring_keyhandler);
> > +
> >      /* Serial input is directed to DOM0 by default. */
> >      switch_serial_input();
> >  }
> 

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