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

I manged to do something similar in the meantime. I am seeing a crash after I do /etc/init.d/xencommns start

[<ffffffc00038fee4>] clear_bit+0x14/0x30
[<ffffffc0003d9ca4>] ack_dynirq+0x44/0x58
[<ffffffc0000e6a34>] handle_edge_irq+0x74/0x178
[<ffffffc0003dc0e8>] evtchn_fifo_handle_events+0x280/0x288
[<ffffffc0003d8f50>] __xen_evtchn_do_upcall+0x68/0xd0
[<ffffffc0003d8fc0>] xen_hvm_evtchn_do_upcall+0x8/0x18
[<ffffffc00009271c>] xen_arm_callback+0x4c/0x68
[<ffffffc0000e7560>] handle_percpu_devid_irq+0x88/0x120
[<ffffffc0000e38b4>] generic_handle_irq+0x24/0x40
[<ffffffc000084890>] handle_IRQ+0x40/0xa8
[<ffffffc000081348>] gic_handle_irq+0x50/0xa0

I found that consume_one_event calls handle_irq_for_port which gets IRQ=7 in case of a crash.
What is the use of IRQ 7 ?
IRQ1 is UART which i saw in cat /proc/interrupts


On 7 August 2014 18:14, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> wrote:
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 @@
Âstatic void __iomem *early_base;
Âstatic void (*printch)(char ch);

+void xen_raw_console_write(const char *str);
 * PL011 single character TX.
Âstatic void pl011_printch(char ch)
- Â Â Â while (readl_relaxed(early_base + UART01x_FR) & UART01x_FR_TXFF)
- Â Â Â Â Â Â Â ;
- Â Â Â writeb_relaxed(ch, early_base + UART01x_DR);
- Â Â Â while (readl_relaxed(early_base + UART01x_FR) & UART01x_FR_BUSY)
- Â Â Â Â Â Â Â ;
+ Â Â Â char buf[2];
+ Â Â Â buf[0] = ch;
+ Â Â Â buf[1] = '\0';
+ Â Â Â 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;

- Â Â Â if (xen_domain()) {
+ Â Â Â if (1||xen_domain()) {
        rc = dom0_write_console(0, str, len);
Â#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);
Âasmlinkage __visible int printk(const char *fmt, ...)
    va_list args;
- Â Â Â int r;
+ Â Â Â static char buf[512];

- Â Â Â if (unlikely(kdb_trap_printk)) {
- Â Â Â Â Â Â Â va_start(args, fmt);
- Â Â Â Â Â Â Â r = vkdb_printf(fmt, args);
- Â Â Â Â Â Â Â va_end(args);
- Â Â Â Â Â Â Â return r;
- Â Â Â }
    va_start(args, fmt);
- Â Â Â r = vprintk_emit(0, -1, NULL, 0, fmt, args);
+ Â Â Â vsnprintf(buf, sizeof(buf), fmt, args);

- Â Â Â return r;
+ Â Â Â xen_raw_console_write(buf);
+ Â Â Â return 1;

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