+}
+EXPORT_SYMBOL_GPL(hv_do_fast_hypercall8);
+
+

nit: Extra line, here and few other places

+u64 hv_get_vpreg(u32 msr)
+{
+	struct hv_get_vp_registers_input	*input;
+	struct hv_get_vp_registers_output	*output;
+	u64					result;

It seems like the code is using both spaces and tabs to align variable names. Can
we stick to one or the other, preferably spaces.

+
+	/*
+	 * Allocate a power of 2 size so alignment to that size is
+	 * guaranteed, since the hypercall input and output areas
+	 * must not cross a page boundary.
+	 */
+	input = kzalloc(roundup_pow_of_two(sizeof(input->header) +
+				sizeof(input->element[0])), GFP_ATOMIC);
+	output = kmalloc(roundup_pow_of_two(sizeof(*output)), GFP_ATOMIC);
+

Check for null from these malloc routines? Here and in other places.

+	__hv_get_vpreg_128(msr, input, output);

+ * Linux-specific definitions for managing interactions with Microsoft's
+ * Hyper-V hypervisor. The definitions in this file are specific to
+ * the ARM64 architecture.  See include/asm-generic/mshyperv.h for
nit: Two space before 'See'. Here and in couple of other places.