[v4,2/2] kvm/cpuid: set proper GuestPhysBits in CPUID.0x80000008

Commit Message

Gerd Hoffmann March 13, 2024, 12:58 p.m. UTC

The AMD APM (3.35) defines GuestPhysBits (EAX[23:16]) as:

  Maximum guest physical address size in bits.  This number applies
  only to guests using nested paging.  When this field is zero, refer
  to the PhysAddrSize field for the maximum guest physical address size.

Tom Lendacky confirmed that the purpose of GuestPhysBits is software use
and KVM can use it as described below.  Hardware always returns zero
here.

Use the GuestPhysBits field to communicate the max addressable GPA to
the guest.  Typically this is identical to the max effective GPA, except
in case the CPU supports MAXPHYADDR > 48 but does not support 5-level
TDP.

GuestPhysBits is set only in case TDP is enabled, otherwise it is left
at zero.

GuestPhysBits will be used by the guest firmware to make sure resources
like PCI bars are mapped into the addressable GPA.

Signed-off-by: Gerd Hoffmann <[e-mail]>
Reviewed-by: Xiaoyao Li <[e-mail]>
---
 arch/x86/kvm/mmu.h     |  2 ++
 arch/x86/kvm/cpuid.c   | 28 +++++++++++++++++++++++++---
 arch/x86/kvm/mmu/mmu.c |  5 +++++
 3 files changed, 32 insertions(+), 3 deletions(-)

Patch

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 60f21bb4c27b..b410a227c601 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -100,6 +100,8 @@  static inline u8 kvm_get_shadow_phys_bits(void)
 	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);
 void kvm_mmu_set_me_spte_mask(u64 me_value, u64 me_mask);
 void kvm_mmu_set_ept_masks(bool has_ad_bits, bool has_exec_only);
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 3235254724cf..b54a85d7387e 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1221,8 +1221,22 @@  static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->eax = entry->ebx = entry->ecx = 0;
 		break;
 	case 0x80000008: {
+		/*
+		 * GuestPhysAddrSize (EAX[23:16]) is intended for software
+		 * use.
+		 *
+		 * KVM's ABI is to report the effective MAXPHYADDR for the
+		 * guest in PhysAddrSize (phys_as), and the maximum
+		 * *addressable* GPA in GuestPhysAddrSize (g_phys_as).
+		 *
+		 * GuestPhysAddrSize is valid if and only if TDP is enabled,
+		 * in which case the max GPA that can be addressed by KVM may
+		 * be less than the max GPA that can be legally generated by
+		 * the guest, e.g. if MAXPHYADDR>48 but the CPU doesn't
+		 * support 5-level TDP.
+		 */
 		unsigned int virt_as = max((entry->eax >> 8) & 0xff, 48U);
-		unsigned int phys_as;
+		unsigned int phys_as, g_phys_as;
 
 		/*
 		 * If TDP (NPT) is disabled use the adjusted host MAXPHYADDR as
@@ -1231,15 +1245,23 @@  static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		 * paging, too.
 		 *
 		 * If TDP is enabled, use the raw bare metal MAXPHYADDR as
-		 * reductions to the HPAs do not affect GPAs.
+		 * reductions to the HPAs do not affect GPAs.  The max
+		 * addressable GPA is the same as the max effective GPA, except
+		 * that it's capped at 48 bits if 5-level TDP isn't supported
+		 * (hardware processes bits 51:48 only when walking the fifth
+		 * level page table).
 		 */
 		if (!tdp_enabled) {
 			phys_as = boot_cpu_data.x86_phys_bits;
+			g_phys_as = 0;
 		} else {
 			phys_as = entry->eax & 0xff;
+			g_phys_as = phys_as;
+			if (kvm_mmu_get_max_tdp_level() < 5)
+				g_phys_as = min(g_phys_as, 48);
 		}
 
-		entry->eax = phys_as | (virt_as << 8);
+		entry->eax = phys_as | (virt_as << 8) | (g_phys_as << 16);
 		entry->ecx &= ~(GENMASK(31, 16) | GENMASK(11, 8));
 		entry->edx = 0;
 		cpuid_entry_override(entry, CPUID_8000_0008_EBX);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 2b515acd8e72..74b9d0354bff 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5309,6 +5309,11 @@  static inline int kvm_mmu_get_tdp_level(struct kvm_vcpu *vcpu)
 	return max_tdp_level;
 }
 
+u8 kvm_mmu_get_max_tdp_level(void)
+{
+	return tdp_root_level ? tdp_root_level : max_tdp_level;
+}
+
 static union kvm_mmu_page_role
 kvm_calc_tdp_mmu_root_page_role(struct kvm_vcpu *vcpu,
 				union kvm_cpu_role cpu_role)