KVM: x86/mmu: Add module param to disable MMIO caching (for testing)

Add a module param to disable MMIO caching so that it's possible to test
the related flows without access to the necessary hardware.  Using shadow
paging with 64-bit KVM and 52 bits of physical address space must disable
MMIO caching as there are no reserved bits to be had.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/spte.c

@@ -17,6 +17,9 @@
 
 #include <asm/e820/api.h>
 
+static bool __read_mostly enable_mmio_caching = true;
+module_param_named(mmio_caching, enable_mmio_caching, bool, 0444);
+
 u64 __read_mostly shadow_nx_mask;
 u64 __read_mostly shadow_x_mask; /* mutual exclusive with nx_mask */
 u64 __read_mostly shadow_user_mask;
@@ -251,6 +254,9 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
 	BUG_ON((u64)(unsigned)access_mask != access_mask);
 	WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);
 
+	if (!enable_mmio_caching)
+		mmio_value = 0;
+
 	/*
 	 * Disable MMIO caching if the MMIO value collides with the bits that
 	 * are used to hold the relocated GFN when the L1TF mitigation is