hyperv: Enable the hypercall output page for the VTL mode

Due to the hypercall page not being allocated in the VTL mode,
the code resorts to using a part of the input page.

Allocate the hypercall output page in the VTL mode thus enabling
it to use it for output and share code with dom0.

Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250108222138.1623703-4-romank@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Message-ID: <20250108222138.1623703-4-romank@linux.microsoft.com>

Author: romank-msft

drivers/hv/hv_common.c

@@ -278,6 +278,11 @@ static void hv_kmsg_dump_register(void)
 	}
 }
 
+static inline bool hv_output_page_exists(void)
+{
+	return hv_root_partition || IS_ENABLED(CONFIG_HYPERV_VTL_MODE);
+}
+
 int __init hv_common_init(void)
 {
 	int i;
@@ -340,7 +345,7 @@ int __init hv_common_init(void)
 	BUG_ON(!hyperv_pcpu_input_arg);
 
 	/* Allocate the per-CPU state for output arg for root */
-	if (hv_root_partition) {
+	if (hv_output_page_exists()) {
 		hyperv_pcpu_output_arg = alloc_percpu(void *);
 		BUG_ON(!hyperv_pcpu_output_arg);
 	}
@@ -435,7 +440,7 @@ int hv_common_cpu_init(unsigned int cpu)
 	void **inputarg, **outputarg;
 	u64 msr_vp_index;
 	gfp_t flags;
-	int pgcount = hv_root_partition ? 2 : 1;
+	const int pgcount = hv_output_page_exists() ? 2 : 1;
 	void *mem;
 	int ret;
 
@@ -453,7 +458,7 @@ int hv_common_cpu_init(unsigned int cpu)
 		if (!mem)
 			return -ENOMEM;
 
-		if (hv_root_partition) {
+		if (hv_output_page_exists()) {
 			outputarg = (void **)this_cpu_ptr(hyperv_pcpu_output_arg);
 			*outputarg = (char *)mem + HV_HYP_PAGE_SIZE;
 		}