[PATCH 2/5] Drivers: hv: Don't assume cpu_possible_mask is dense

[mhkelley58@gmail.com]

From: Michael Kelley <mhklinux@outlook.com>

Current code allocates the hv_vp_index array with size
num_possible_cpus(). This code assumes cpu_possible_mask is dense,
which is not true in the general case per [1]. If cpu_possible_mask
is sparse, the array might be indexed by a value beyond the size of
the array.

However, the configurations that Hyper-V provides to guest VMs on x86
and ARM64 hardware, in combination with how architecture specific code
assigns Linux CPU numbers, *does* always produce a dense cpu_possible_mask.
So the dense assumption is not currently causing failures. But for
robustness against future changes in how cpu_possible_mask is populated,
update the code to no longer assume dense.

The correct approach is to allocate and initialize the array using size
"nr_cpu_ids". While this leaves unused array entries corresponding to
holes in cpu_possible_mask, the holes are assumed to be minimal and hence
the amount of memory wasted by unused entries is minimal.

Using nr_cpu_ids also reduces initialization time, in that the loop to
initialize the array currently rescans cpu_possible_mask on each
iteration. This is n-squared in the number of CPUs, which could be
significant for large CPU counts.

[1] https://lore.kernel.org/lkml/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
---
 drivers/hv/hv_common.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
index d50caf0d723d..8c44938cb084 100644
--- a/drivers/hv/hv_common.c
+++ b/drivers/hv/hv_common.c
@@ -345,14 +345,14 @@ int __init hv_common_init(void)
 		BUG_ON(!hyperv_pcpu_output_arg);
 	}
 
-	hv_vp_index = kmalloc_array(num_possible_cpus(), sizeof(*hv_vp_index),
+	hv_vp_index = kmalloc_array(nr_cpu_ids, sizeof(*hv_vp_index),
 				    GFP_KERNEL);
 	if (!hv_vp_index) {
 		hv_common_free();
 		return -ENOMEM;
 	}
 
-	for (i = 0; i < num_possible_cpus(); i++)
+	for (i = 0; i < nr_cpu_ids; i++)
 		hv_vp_index[i] = VP_INVAL;
 
 	return 0;
-- 
2.25.1