KVM: x86: Avoid calling kvm_is_mmio_pfn() when kvm_x86_ops.get_mt_mask is NULL

Guard the call to kvm_x86_call(get_mt_mask) with an explicit check on
kvm_x86_ops.get_mt_mask so as to avoid unnecessarily calling
kvm_is_mmio_pfn(), which is moderately expensive for some backing types.
E.g. lookup_memtype() conditionally takes a system-wide spinlock if KVM
ends up being call pat_pfn_immune_to_uc_mtrr(), e.g. for DAX memory.

While the call to kvm_x86_ops.get_mt_mask() itself is elided, the compiler
still needs to compute all parameters, as it can't know at build time that
the call will be squashed.

   <+243>:   call   0xffffffff812ad880 <kvm_is_mmio_pfn>
   <+248>:   mov    %r13,%rsi
   <+251>:   mov    %rbx,%rdi
   <+254>:   movzbl %al,%edx
   <+257>:   call   0xffffffff81c26af0 <__SCT__kvm_x86_get_mt_mask>

Fixes: 3fee483 ("KVM: x86: remove shadow_memtype_mask")
Tested-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://lore.kernel.org/r/20250523011756.3243624-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/mmu/spte.c

@@ -209,7 +209,9 @@ bool make_spte(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
 	if (level > PG_LEVEL_4K)
 		spte |= PT_PAGE_SIZE_MASK;
 
-	spte |= kvm_x86_call(get_mt_mask)(vcpu, gfn, kvm_is_mmio_pfn(pfn));
+	if (kvm_x86_ops.get_mt_mask)
+		spte |= kvm_x86_call(get_mt_mask)(vcpu, gfn, kvm_is_mmio_pfn(pfn));
+
 	if (host_writable)
 		spte |= shadow_host_writable_mask;
 	else