Add a pointer hv_synic_eventring_tail to track the tail pointer for the
SynIC event ring buffer for each SINT.

This will be used by the mshv driver, but must be tracked independently
since the driver module could be removed and re-inserted.

Signed-off-by: Nuno Das Neves <redacted>
Reviewed-by: Wei Liu <wei.liu@kernel.org>
---
 drivers/hv/hv_common.c | 34 ++++++++++++++++++++++++++++++++--
 1 file changed, 32 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index 252fd66ad4db..2763cb6d3678 100644
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c

@@ -460,6 +478,7 @@ void __init ms_hyperv_late_init(void)
 int hv_common_cpu_init(unsigned int cpu)
 {
 	void **inputarg, **outputarg;
+	u8 **synic_eventring_tail;
 	u64 msr_vp_index;
 	gfp_t flags;
 	const int pgcount = hv_output_page_exists() ? 2 : 1;

@@ -472,8 +491,8 @@ int hv_common_cpu_init(unsigned int cpu)
 	inputarg = (void **)this_cpu_ptr(hyperv_pcpu_input_arg);
 
 	/*
-	 * hyperv_pcpu_input_arg and hyperv_pcpu_output_arg memory is already
-	 * allocated if this CPU was previously online and then taken offline
+	 * The per-cpu memory is already allocated if this CPU was previously
+	 * online and then taken offline
 	 */
 	if (!*inputarg) {
 		mem = kmalloc(pgcount * HV_HYP_PAGE_SIZE, flags);

@@ -485,6 +504,17 @@ int hv_common_cpu_init(unsigned int cpu)
 			*outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
 		}
 
+		if (hv_root_partition()) {
+			synic_eventring_tail = (u8 **)this_cpu_ptr(hv_synic_eventring_tail);
+			*synic_eventring_tail = kcalloc(HV_SYNIC_SINT_COUNT,
+							sizeof(u8), flags);
+