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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

Also CC Linux maintainers.

On Tue, Sep 04, 2018 at 07:27:31PM +0200, Vitaly Kuznetsov wrote:
> Wei Liu <wei.liu2@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > Hmm... this node isn't documented.
> >
> 
> But libxl already writes it, see libxl_send_sysrq().

I think your patch is fine.

What I wanted to do is to document this node. :p

> 
> > Can you point me to the code in Linux?
> >
> 
> Sure,
> 
> drivers/xen/manage.c:

Alright, it appears DomU only expects a single character to be written.

I have written the following diff, do you think it is sensible? I
believe this is Linux only, but I have CC Roger for correction.


diff --git a/docs/misc/xenstore-paths.markdown 
b/docs/misc/xenstore-paths.markdown
index 60c8b3fbe5..33d281915c 100644
--- a/docs/misc/xenstore-paths.markdown
+++ b/docs/misc/xenstore-paths.markdown
@@ -410,6 +410,14 @@ by udev ("0") or will be run by the toolstack directly 
("1").
 
 ### Platform Feature and Control Paths
 
+#### ~/control/sysrq = (""|COMMAND) [w]
+
+This is the PV SysRq control node. A toolstack can write a single character
+representing a magic SysRq key understood by the Linux kernel.  The guest
+acknowledges a request by writing the empty string back to the command node.
+
+This protocol is Linux only.
+
 #### ~/control/shutdown = (""|COMMAND) [w]
 
 This is the PV shutdown control node. A toolstack can write various


Wei.

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