Re: [PATCH] drivers/xen: Improve the late XenStore init protocol

[Henry Wang]

Hi Stefano,

On 5/17/2024 8:52 AM, Stefano Stabellini wrote:
On Thu, 16 May 2024, Henry Wang wrote:
   enum xenstore_init xen_store_domain_type;
   EXPORT_SYMBOL_GPL(xen_store_domain_type);
   @@ -751,9 +755,10 @@ static void xenbus_probe(void)
   {
        xenstored_ready = 1;
   -    if (!xen_store_interface) {
-               xen_store_interface = memremap(xen_store_gfn <<
XEN_PAGE_SHIFT,
-                                              XEN_PAGE_SIZE, MEMREMAP_WB);
+       if (!xen_store_interface || XS_INTERFACE_READY) {
+               if (!xen_store_interface)
These two nested if's don't make sense to me. If XS_INTERFACE_READY
succeeds, it means that  ((xen_store_interface != NULL) &&
(xen_store_interface->connection == XENSTORE_CONNECTED)).
So it is not possible that xen_store_interface == NULL immediately
after. Right?
I think this is because we want to free the irq for the late init case,
otherwise the init-dom0less will fail. For the xenstore PFN allocated case,
the connection is already set to CONNECTED when we execute init-dom0less. But
I agree with you, would below diff makes more sense to you?

diff --git a/drivers/xen/xenbus/xenbus_probe.c
b/drivers/xen/xenbus/xenbus_probe.c
index 8aec0ed1d047..b8005b651a29 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -76,6 +76,8 @@ EXPORT_SYMBOL_GPL(xen_store_interface);
         ((xen_store_interface != NULL) && \
          (xen_store_interface->connection == XENSTORE_CONNECTED))

+static bool xs_late_init = false;
+
  enum xenstore_init xen_store_domain_type;
  EXPORT_SYMBOL_GPL(xen_store_domain_type);

@@ -755,7 +757,7 @@ static void xenbus_probe(void)
  {
         xenstored_ready = 1;

-       if (!xen_store_interface || XS_INTERFACE_READY) {
+       if (xs_late_init) {
                 if (!xen_store_interface)
                         xen_store_interface = memremap(xen_store_gfn <<

I would just remove the outer 'if' and do this:


        if (!xen_store_interface)
                xen_store_interface = memremap(xen_store_gfn << XEN_PAGE_SHIFT,
                                XEN_PAGE_SIZE, MEMREMAP_WB);
        /*
         * Now it is safe to free the IRQ used for xenstore late
         * initialization. No need to unbind: it is about to be
         * bound again from xb_init_comms. Note that calling
         * unbind_from_irqhandler now would result in xen_evtchn_close()
         * being called and the event channel not being enabled again
         * afterwards, resulting in missed event notifications.
         */
        if (xs_init_irq > 0)
                free_irq(xs_init_irq, &xb_waitq);


I think this should work fine in all cases.

Thanks. I followed your suggestion in v2.

  I am unsure if
xs_init_irq==0 is possible valid value for xs_init_irq. If it is not,
then we are fine. If 0 is a possible valid irq number, then we should
initialize xs_init_irq to -1, and here check for xs_init_irq >= 0.

Yeah the xs_init_irq==0 is a valid value. I followed your latter comment 
to init it to -1 and check it >=0.

Kind regards,
Henry