Drivers: hv: vmbus: Fix vmbus_wait_for_unload() to scan present CPUs

vmbus_wait_for_unload() may be called in the panic path after other
CPUs are stopped. vmbus_wait_for_unload() currently loops through
online CPUs looking for the UNLOAD response message. But the values of
CONFIG_KEXEC_CORE and crash_kexec_post_notifiers affect the path used
to stop the other CPUs, and in one of the paths the stopped CPUs
are removed from cpu_online_mask. This removal happens in both
x86/x64 and arm64 architectures. In such a case, vmbus_wait_for_unload()
only checks the panic'ing CPU, and misses the UNLOAD response message
except when the panic'ing CPU is CPU 0. vmbus_wait_for_unload()
eventually times out, but only after waiting 100 seconds.

Fix this by looping through *present* CPUs in vmbus_wait_for_unload().
The cpu_present_mask is not modified by stopping the other CPUs in the
panic path, nor should it be.

Also, in a CoCo VM the synic_message_page is not allocated in
hv_synic_alloc(), but is set and cleared in hv_synic_enable_regs()
and hv_synic_disable_regs() such that it is set only when the CPU is
online.  If not all present CPUs are online when vmbus_wait_for_unload()
is called, the synic_message_page might be NULL. Add a check for this.

Fixes: cd95aad55793 ("Drivers: hv: vmbus: handle various crash scenarios")
Cc: stable@vger.kernel.org
Reported-by: John Starks <jostarks@microsoft.com>
Signed-off-by: Michael Kelley <mikelley@microsoft.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/1684422832-38476-1-git-send-email-mikelley@microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>

diff --git a/drivers/hv/channel_mgmt.c b/drivers/hv/channel_mgmt.c
index 007f26d..2f4d09c 100644 (file)
--- a/drivers/hv/channel_mgmt.c
+++ b/drivers/hv/channel_mgmt.c
@@ -829,11 +829,22 @@ static void vmbus_wait_for_unload(void)
                if (completion_done(&vmbus_connection.unload_event))
                        goto completed;
 
-               for_each_online_cpu(cpu) {
+               for_each_present_cpu(cpu) {
                        struct hv_per_cpu_context *hv_cpu
                                = per_cpu_ptr(hv_context.cpu_context, cpu);
 
+                       /*
+                        * In a CoCo VM the synic_message_page is not allocated
+                        * in hv_synic_alloc(). Instead it is set/cleared in
+                        * hv_synic_enable_regs() and hv_synic_disable_regs()
+                        * such that it is set only when the CPU is online. If
+                        * not all present CPUs are online, the message page
+                        * might be NULL, so skip such CPUs.
+                        */
                        page_addr = hv_cpu->synic_message_page;
+                       if (!page_addr)
+                               continue;
+
                        msg = (struct hv_message *)page_addr
                                + VMBUS_MESSAGE_SINT;
 
@@ -867,11 +878,14 @@ completed:
         * maybe-pending messages on all CPUs to be able to receive new
         * messages after we reconnect.
         */
-       for_each_online_cpu(cpu) {
+       for_each_present_cpu(cpu) {
                struct hv_per_cpu_context *hv_cpu
                        = per_cpu_ptr(hv_context.cpu_context, cpu);
 
                page_addr = hv_cpu->synic_message_page;
+               if (!page_addr)
+                       continue;
+
                msg = (struct hv_message *)page_addr + VMBUS_MESSAGE_SINT;
                msg->header.message_type = HVMSG_NONE;
        }