drm/i915/gvt: treat intel_gvt_mpt as const in gvt code

The current interface of intel_gvt_register_hypervisor() expects a
non-const pointer to struct intel_gvt_mpt, even though the mediator
never modifies (or should modifiy) the content of this struct.

Change the function signature and relevant struct members to const to
properly express the API's intent and allow instances of intel_gvt_mpt
to be allocated as const.

While I was here, I also made KVM's instance of this struct const to
reduce the number of writable function pointers in the kernel.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: intel-gvt-dev@lists.freedesktop.org
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Julian Stecklina <julian.stecklina@cyberus-technology.de>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20201111172811.558443-1-julian.stecklina@cyberus-technology.de

drivers/gpu/drm/i915/gvt/gvt.c

@@ -415,7 +415,7 @@ intel_gvt_pm_resume(struct intel_gvt *gvt)
 }
 
 int
-intel_gvt_register_hypervisor(struct intel_gvt_mpt *m)
+intel_gvt_register_hypervisor(const struct intel_gvt_mpt *m)
 {
 	int ret;
 	void *gvt;

drivers/gpu/drm/i915/gvt/gvt.h

@@ -56,7 +56,7 @@ struct intel_gvt_host {
 	struct device *dev;
 	bool initialized;
 	int hypervisor_type;
-	struct intel_gvt_mpt *mpt;
+	const struct intel_gvt_mpt *mpt;
 };
 
 extern struct intel_gvt_host intel_gvt_host;

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -2099,7 +2099,7 @@ static bool kvmgt_is_valid_gfn(unsigned long handle, unsigned long gfn)
 	return ret;
 }
 
-static struct intel_gvt_mpt kvmgt_mpt = {
+static const struct intel_gvt_mpt kvmgt_mpt = {
 	.type = INTEL_GVT_HYPERVISOR_KVM,
 	.host_init = kvmgt_host_init,
 	.host_exit = kvmgt_host_exit,

drivers/gpu/drm/i915/gvt/mpt.h

@@ -392,7 +392,7 @@ static inline bool intel_gvt_hypervisor_is_valid_gfn(
 	return intel_gvt_host.mpt->is_valid_gfn(vgpu->handle, gfn);
 }
 
-int intel_gvt_register_hypervisor(struct intel_gvt_mpt *);
+int intel_gvt_register_hypervisor(const struct intel_gvt_mpt *);
 void intel_gvt_unregister_hypervisor(void);
 
 #endif /* _GVT_MPT_H_ */