Re: [PATCH] libxl: create control/sysrq xenstore node

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Wei Liu <wei.liu2@citrix.com> writes:

> On Tue, Sep 04, 2018 at 01:39:29PM +0200, Vitaly Kuznetsov wrote:
>> 'xl sysrq' command doesn't work with modern Linux guests with the following
>> message in guest's log:
>> 
>>  xen:manage: sysrq_handler: Error -13 writing sysrq in control/sysrq
>> 
>> xenstore trace confirms:
>> 
>>  IN 0x24bd9a0 20180904 04:36:32 WRITE (control/sysrq )
>>  OUT 0x24bd9a0 20180904 04:36:32 ERROR (EACCES )
>> 
>> The problem seems to be in the fact that we don't pre-create control/sysrq
>> xenstore node and libxl_send_sysrq() doing libxl__xs_printf() creates it as
>> read-only. As we want to allow guests to clean 'control/sysrq' after the
>> requested action is performed, we need to make this node writable.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> Hmm... this node isn't documented.
>

But libxl already writes it, see libxl_send_sysrq().

> Can you point me to the code in Linux?
>

Sure,

drivers/xen/manage.c:

static void sysrq_handler(struct xenbus_watch *watch, const char *path,
			  const char *token)
{
	char sysrq_key = '\0';
	struct xenbus_transaction xbt;
	int err;

 again:
	err = xenbus_transaction_start(&xbt);
	if (err)
		return;
	err = xenbus_scanf(xbt, "control", "sysrq", "%c", &sysrq_key);
	if (err < 0) {
		/*
		 * The Xenstore watch fires directly after registering it and
		 * after a suspend/resume cycle. So ENOENT is no error but
		 * might happen in those cases.
		 */
		if (err != -ENOENT)
			pr_err("Error %d reading sysrq code in control/sysrq\n",
			       err);
		xenbus_transaction_end(xbt, 1);
		return;
	}

	if (sysrq_key != '\0') {
		err = xenbus_printf(xbt, "control", "sysrq", "%c", '\0');
		if (err) {
			pr_err("%s: Error %d writing sysrq in control/sysrq\n",
			       __func__, err);
			xenbus_transaction_end(xbt, 1);
			return;
		}
	}

	err = xenbus_transaction_end(xbt, 0);
	if (err == -EAGAIN)
		goto again;

	if (sysrq_key != '\0')
		handle_sysrq(sysrq_key);
}

static struct xenbus_watch sysrq_watch = {
	.node = "control/sysrq",
	.callback = sysrq_handler
};


-- 
  Vitaly

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