KVM: x86/mmu: Snapshot shadow_phys_bits when kvm.ko is loaded

Snapshot shadow_phys_bits when kvm.ko is loaded, not when a vendor module
is loaded, to guard against usage of shadow_phys_bits before it is
initialized.  The computation isn't vendor specific in any way, i.e. there
there is no reason to wait to snapshot the value until a vendor module is
loaded, nor is there any reason to recompute the value every time a vendor
module is loaded.

Opportunistically convert it from "read mostly" to "read-only after init".

Link: https://lore.kernel.org/r/20240423221521.2923759-4-seanjc@google.comSigned-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu.h

@@ -61,7 +61,7 @@ static __always_inline u64 rsvd_bits(int s, int e)
  * The number of non-reserved physical address bits irrespective of features
  * that repurpose legal bits, e.g. MKTME.
  */
-extern u8 __read_mostly shadow_phys_bits;
+extern u8 __ro_after_init shadow_phys_bits;
 
 static inline gfn_t kvm_mmu_max_gfn(void)
 {

arch/x86/kvm/mmu/spte.c

@@ -43,7 +43,7 @@ u64 __read_mostly shadow_acc_track_mask;
 u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
 u64 __read_mostly shadow_nonpresent_or_rsvd_lower_gfn_mask;
 
-u8 __read_mostly shadow_phys_bits;
+u8 __ro_after_init shadow_phys_bits;
 
 void __init kvm_mmu_spte_module_init(void)
 {
@@ -55,6 +55,8 @@ void __init kvm_mmu_spte_module_init(void)
 	 * will change when the vendor module is (re)loaded.
 	 */
 	allow_mmio_caching = enable_mmio_caching;
+
+	shadow_phys_bits = kvm_get_shadow_phys_bits();
 }
 
 static u64 generation_mmio_spte_mask(u64 gen)
@@ -441,8 +443,6 @@ void kvm_mmu_reset_all_pte_masks(void)
 	u8 low_phys_bits;
 	u64 mask;
 
-	shadow_phys_bits = kvm_get_shadow_phys_bits();
-
 	/*
 	 * If the CPU has 46 or less physical address bits, then set an
 	 * appropriate mask to guard against L1TF attacks. Otherwise, it is