KVM: x86: Move shadow_phys_bits into "kvm_host", as "maxphyaddr"

Move shadow_phys_bits into "struct kvm_host_values", i.e. into KVM's
global "kvm_host" variable, so that it is automatically exported for use
in vendor modules.  Rename the variable/field to maxphyaddr to more
clearly capture what value it holds, now that it's used outside of the
MMU (and because the "shadow" part is more than a bit misleading as the
variable is not at all unique to shadow paging).

Recomputing the raw/true host.MAXPHYADDR on every use can be subtly
expensive, e.g. it will incur a VM-Exit on the CPUID if KVM is running as
a nested hypervisor.  Vendor code already has access to the information,
e.g. by directly doing CPUID or by invoking kvm_get_shadow_phys_bits(), so
there's no tangible benefit to making it MMU-only.

Link: https://lore.kernel.org/r/20240423221521.2923759-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/mmu.h

@@ -57,12 +57,6 @@ static __always_inline u64 rsvd_bits(int s, int e)
 	return ((2ULL << (e - s)) - 1) << s;
 }
 
-/*
- * The number of non-reserved physical address bits irrespective of features
- * that repurpose legal bits, e.g. MKTME.
- */
-extern u8 __ro_after_init shadow_phys_bits;
-
 static inline gfn_t kvm_mmu_max_gfn(void)
 {
 	/*
@@ -76,30 +70,11 @@ static inline gfn_t kvm_mmu_max_gfn(void)
 	 * than hardware's real MAXPHYADDR.  Using the host MAXPHYADDR
 	 * disallows such SPTEs entirely and simplifies the TDP MMU.
 	 */
-	int max_gpa_bits = likely(tdp_enabled) ? shadow_phys_bits : 52;
+	int max_gpa_bits = likely(tdp_enabled) ? kvm_host.maxphyaddr : 52;
 
 	return (1ULL << (max_gpa_bits - PAGE_SHIFT)) - 1;
 }
 
-static inline u8 kvm_get_shadow_phys_bits(void)
-{
-	/*
-	 * boot_cpu_data.x86_phys_bits is reduced when MKTME or SME are detected
-	 * in CPU detection code, but the processor treats those reduced bits as
-	 * 'keyID' thus they are not reserved bits. Therefore KVM needs to look at
-	 * the physical address bits reported by CPUID.
-	 */
-	if (likely(boot_cpu_data.extended_cpuid_level >= 0x80000008))
-		return cpuid_eax(0x80000008) & 0xff;
-
-	/*
-	 * Quite weird to have VMX or SVM but not MAXPHYADDR; probably a VM with
-	 * custom CPUID.  Proceed with whatever the kernel found since these features
-	 * aren't virtualizable (SME/SEV also require CPUIDs higher than 0x80000008).
-	 */
-	return boot_cpu_data.x86_phys_bits;
-}
-
 u8 kvm_mmu_get_max_tdp_level(void);
 
 void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask);

arch/x86/kvm/mmu/mmu.c

@@ -4980,7 +4980,7 @@ static void reset_rsvds_bits_mask_ept(struct kvm_vcpu *vcpu,
 
 static inline u64 reserved_hpa_bits(void)
 {
-	return rsvd_bits(shadow_phys_bits, 63);
+	return rsvd_bits(kvm_host.maxphyaddr, 63);
 }
 
 /*

arch/x86/kvm/mmu/spte.c

@@ -43,7 +43,25 @@ 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 __ro_after_init shadow_phys_bits;
+static u8 __init kvm_get_host_maxphyaddr(void)
+{
+	/*
+	 * boot_cpu_data.x86_phys_bits is reduced when MKTME or SME are detected
+	 * in CPU detection code, but the processor treats those reduced bits as
+	 * 'keyID' thus they are not reserved bits. Therefore KVM needs to look at
+	 * the physical address bits reported by CPUID, i.e. the raw MAXPHYADDR,
+	 * when reasoning about CPU behavior with respect to MAXPHYADDR.
+	 */
+	if (likely(boot_cpu_data.extended_cpuid_level >= 0x80000008))
+		return cpuid_eax(0x80000008) & 0xff;
+
+	/*
+	 * Quite weird to have VMX or SVM but not MAXPHYADDR; probably a VM with
+	 * custom CPUID.  Proceed with whatever the kernel found since these features
+	 * aren't virtualizable (SME/SEV also require CPUIDs higher than 0x80000008).
+	 */
+	return boot_cpu_data.x86_phys_bits;
+}
 
 void __init kvm_mmu_spte_module_init(void)
 {
@@ -56,7 +74,7 @@ void __init kvm_mmu_spte_module_init(void)
 	 */
 	allow_mmio_caching = enable_mmio_caching;
 
-	shadow_phys_bits = kvm_get_shadow_phys_bits();
+	kvm_host.maxphyaddr = kvm_get_host_maxphyaddr();
 }
 
 static u64 generation_mmio_spte_mask(u64 gen)
@@ -494,7 +512,7 @@ void kvm_mmu_reset_all_pte_masks(void)
 	 * 52-bit physical addresses then there are no reserved PA bits in the
 	 * PTEs and so the reserved PA approach must be disabled.
 	 */
-	if (shadow_phys_bits < 52)
+	if (kvm_host.maxphyaddr < 52)
 		mask = BIT_ULL(51) | PT_PRESENT_MASK;
 	else
 		mask = 0;

arch/x86/kvm/vmx/vmx.c

@@ -8389,18 +8389,16 @@ static void __init vmx_setup_me_spte_mask(void)
 	u64 me_mask = 0;
 
 	/*
-	 * kvm_get_shadow_phys_bits() returns shadow_phys_bits.  Use
-	 * the former to avoid exposing shadow_phys_bits.
-	 *
 	 * On pre-MKTME system, boot_cpu_data.x86_phys_bits equals to
-	 * shadow_phys_bits.  On MKTME and/or TDX capable systems,
+	 * kvm_host.maxphyaddr.  On MKTME and/or TDX capable systems,
 	 * boot_cpu_data.x86_phys_bits holds the actual physical address
-	 * w/o the KeyID bits, and shadow_phys_bits equals to MAXPHYADDR
-	 * reported by CPUID.  Those bits between are KeyID bits.
+	 * w/o the KeyID bits, and kvm_host.maxphyaddr equals to
+	 * MAXPHYADDR reported by CPUID.  Those bits between are KeyID bits.
 	 */
-	if (boot_cpu_data.x86_phys_bits != kvm_get_shadow_phys_bits())
+	if (boot_cpu_data.x86_phys_bits != kvm_host.maxphyaddr)
 		me_mask = rsvd_bits(boot_cpu_data.x86_phys_bits,
-			kvm_get_shadow_phys_bits() - 1);
+				    kvm_host.maxphyaddr - 1);
+
 	/*
 	 * Unlike SME, host kernel doesn't support setting up any
 	 * MKTME KeyID on Intel platforms.  No memory encryption

arch/x86/kvm/vmx/vmx.h

@@ -727,7 +727,7 @@ static inline bool vmx_need_pf_intercept(struct kvm_vcpu *vcpu)
 		return true;
 
 	return allow_smaller_maxphyaddr &&
-	       cpuid_maxphyaddr(vcpu) < kvm_get_shadow_phys_bits();
+	       cpuid_maxphyaddr(vcpu) < kvm_host.maxphyaddr;
 }
 
 static inline bool is_unrestricted_guest(struct kvm_vcpu *vcpu)

arch/x86/kvm/x86.h

@@ -34,6 +34,13 @@ struct kvm_caps {
 };
 
 struct kvm_host_values {
+	/*
+	 * The host's raw MAXPHYADDR, i.e. the number of non-reserved physical
+	 * address bits irrespective of features that repurpose legal bits,
+	 * e.g. MKTME.
+	 */
+	u8 maxphyaddr;
+
 	u64 efer;
 	u64 xcr0;
 	u64 xss;