x86/hyperv: Don't assume cpu_possible_mask is dense

Current code allocates the hv_vp_assist_page 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
hardware, in combination with how x86 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 the array with 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.

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

Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241003035333.49261-2-mhklinux@outlook.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20241003035333.49261-2-mhklinux@outlook.com>

arch/x86/hyperv/hv_init.c

@@ -472,7 +472,7 @@ void __init hyperv_init(void)
 	if (hv_isolation_type_tdx())
 		hv_vp_assist_page = NULL;
 	else
-		hv_vp_assist_page = kcalloc(num_possible_cpus(),
+		hv_vp_assist_page = kcalloc(nr_cpu_ids,
 					    sizeof(*hv_vp_assist_page),
 					    GFP_KERNEL);
 	if (!hv_vp_assist_page) {