Re: [Xen-devel] How to display dom0 kernel printk on hvc0

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Thu, 7 Aug 2014, manish jaggi wrote:
> Hi Stefano,
> 
> I am debugging a kernel crash when I start xencommons. If I add prints in 
> kernel the prints are only visible in
> dmesg and not on dom0 console. This makes it difficult to view kernel printks 
> at the time of crash.
> 
> I tried xen_raw_printks but the print only comes when you press a key, so 
> while a crash is happening you cannot
> press a key.
> 
> Is there a way to enable printks on hvc0Â
> OR
> is there a way to dump __logbuf of linux from xen shell.

You can hack printk and/or early_printk so that everything will go via
the Xen console, see below. Of course this is very hacky, don't tell
anybody that I wrote this patch ;-)


diff --git a/arch/arm64/kernel/early_printk.c b/arch/arm64/kernel/early_printk.c
index 2dc36d0..38bcf29 100644
--- a/arch/arm64/kernel/early_printk.c
+++ b/arch/arm64/kernel/early_printk.c
@@ -31,16 +31,16 @@
 static void __iomem *early_base;
 static void (*printch)(char ch);
 
+void xen_raw_console_write(const char *str);
 /*
  * PL011 single character TX.
  */
 static void pl011_printch(char ch)
 {
-       while (readl_relaxed(early_base + UART01x_FR) & UART01x_FR_TXFF)
-               ;
-       writeb_relaxed(ch, early_base + UART01x_DR);
-       while (readl_relaxed(early_base + UART01x_FR) & UART01x_FR_BUSY)
-               ;
+       char buf[2];
+       buf[0] = ch;
+       buf[1] = '\0';
+       xen_raw_console_write(buf);
 }
 
 /*
diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
index 2dc2831..1f586b3 100644
--- a/drivers/tty/hvc/hvc_xen.c
+++ b/drivers/tty/hvc/hvc_xen.c
@@ -630,7 +630,7 @@ void xen_raw_console_write(const char *str)
        ssize_t len = strlen(str);
        int rc = 0;
 
-       if (xen_domain()) {
+       if (1||xen_domain()) {
                rc = dom0_write_console(0, str, len);
 #ifdef CONFIG_X86
                if (rc == -ENOSYS && xen_hvm_domain())
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7228258..e035871 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1674,24 +1674,18 @@ EXPORT_SYMBOL(printk_emit);
  *
  * See the vsnprintf() documentation for format string extensions over C99.
  */
+void xen_raw_console_write(const char *str);
 asmlinkage __visible int printk(const char *fmt, ...)
 {
        va_list args;
-       int r;
+       static char buf[512];
 
-#ifdef CONFIG_KGDB_KDB
-       if (unlikely(kdb_trap_printk)) {
-               va_start(args, fmt);
-               r = vkdb_printf(fmt, args);
-               va_end(args);
-               return r;
-       }
-#endif
        va_start(args, fmt);
-       r = vprintk_emit(0, -1, NULL, 0, fmt, args);
+       vsnprintf(buf, sizeof(buf), fmt, args);
        va_end(args);
 
-       return r;
+       xen_raw_console_write(buf);
+       return 1;
 }
 EXPORT_SYMBOL(printk);

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