[PATCH v3 09/62] KVM: SVM: Drop pointless masking of kernel page pa's with AVIC HPA masks

Sean Christopherson
2025-06-11T22:47:12Z

Drop AVIC_HPA_MASK and all its users, the mask is just the 4KiB-aligned
maximum theoretical physical address for x86-64 CPUs, as x86-64 is
currently defined (going beyond PA52 would require an entirely new paging
mode, which would arguably create a new, different architecture).

All usage in KVM masks the result of page_to_phys(), which on x86-64 is
guaranteed to be 4KiB aligned and a legal physical address; if either of
those requirements doesn't hold true, KVM has far bigger problems.

Drop masking the avic_backing_page with
AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK for all the same reasons, but
keep the macro even though it's unused in functional code. It's a
distinct architectural define, and having the definition in software
helps visualize the layout of an entry. And to be hyper-paranoid about
MAXPA going beyond 52, add a compile-time assert to ensure the kernel's
maximum supported physical address stays in bounds.

The unnecessary masking in avic_init_vmcb() also incorrectly assumes that
SME's C-bit resides between bits 51:11; that holds true for current CPUs,
but isn't required by AMD's architecture:

In some implementations, the bit used may be a physical address bit

Key word being "may".

Opportunistically use the GENMASK_ULL() version for
AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK, which is far more readable
than a set of repeating Fs.

Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/include/asm/svm.h | 4 +---
arch/x86/kvm/svm/avic.c | 18 ++++++++++--------
2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/svm.h b/arch/x86/include/asm/svm.h
index 89a666952b01..36f67c69ea66 100644
--- a/arch/x86/include/asm/svm.h
+++ b/arch/x86/include/asm/svm.h
@@ -253,7 +253,7 @@ struct __attribute__ ((__packed__)) vmcb_control_area {
#define AVIC_LOGICAL_ID_ENTRY_VALID_MASK (1 << 31)

#define AVIC_PHYSICAL_ID_ENTRY_HOST_PHYSICAL_ID_MASK GENMASK_ULL(11, 0)
-#define AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK (0xFFFFFFFFFFULL << 12)
+#define AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK GENMASK_ULL(51, 12)
#define AVIC_PHYSICAL_ID_ENTRY_IS_RUNNING_MASK (1ULL << 62)
#define AVIC_PHYSICAL_ID_ENTRY_VALID_MASK (1ULL << 63)
#define AVIC_PHYSICAL_ID_TABLE_SIZE_MASK (0xFFULL)
@@ -288,8 +288,6 @@ enum avic_ipi_failure_cause {
static_assert((AVIC_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == AVIC_MAX_PHYSICAL_ID);
static_assert((X2AVIC_MAX_PHYSICAL_ID & AVIC_PHYSICAL_MAX_INDEX_MASK) == X2AVIC_MAX_PHYSICAL_ID);

-#define AVIC_HPA_MASK ~((0xFFFULL << 52) | 0xFFF)
-
#define SVM_SEV_FEAT_SNP_ACTIVE BIT(0)
#define SVM_SEV_FEAT_RESTRICTED_INJECTION BIT(3)
#define SVM_SEV_FEAT_ALTERNATE_INJECTION BIT(4)
diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
index 5344ae76c590..4b882148f2c0 100644
--- a/arch/x86/kvm/svm/avic.c
+++ b/arch/x86/kvm/svm/avic.c
@@ -241,9 +241,9 @@ void avic_init_vmcb(struct vcpu_svm *svm, struct vmcb *vmcb)
phys_addr_t lpa = __sme_set(page_to_phys(kvm_svm->avic_logical_id_table_page));
phys_addr_t ppa = __sme_set(page_to_phys(kvm_svm->avic_physical_id_table_page));

- vmcb->control.avic_backing_page = bpa & AVIC_HPA_MASK;
- vmcb->control.avic_logical_id = lpa & AVIC_HPA_MASK;
- vmcb->control.avic_physical_id = ppa & AVIC_HPA_MASK;
+ vmcb->control.avic_backing_page = bpa;
+ vmcb->control.avic_logical_id = lpa;
+ vmcb->control.avic_physical_id = ppa;
vmcb->control.avic_vapic_bar = APIC_DEFAULT_PHYS_BASE;

if (kvm_apicv_activated(svm->vcpu.kvm))
@@ -301,9 +301,12 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
if (!entry)
return -EINVAL;

- new_entry = __sme_set((page_to_phys(svm->avic_backing_page) &
- AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK) |
- AVIC_PHYSICAL_ID_ENTRY_VALID_MASK);
+ /* Note, fls64() returns the bit position, +1. */
+ BUILD_BUG_ON(__PHYSICAL_MASK_SHIFT >
+ fls64(AVIC_PHYSICAL_ID_ENTRY_BACKING_PAGE_MASK));
+
+ new_entry = __sme_set(page_to_phys(svm->avic_backing_page)) |
+ AVIC_PHYSICAL_ID_ENTRY_VALID_MASK;
WRITE_ONCE(*entry, new_entry);

svm->avic_physical_id_cache = entry;
@@ -903,8 +906,7 @@ int avic_pi_update_irte(struct kvm_kernel_irqfd *irqfd, struct kvm *kvm,
enable_remapped_mode = false;

/* Try to enable guest_mode in IRTE */
- pi.base = __sme_set(page_to_phys(svm->avic_backing_page) &
- AVIC_HPA_MASK);
+ pi.base = __sme_set(page_to_phys(svm->avic_backing_page));
pi.ga_tag = AVIC_GATAG(to_kvm_svm(kvm)->avic_vm_id,
svm->vcpu.vcpu_id);
pi.is_guest_mode = true;
--
2.50.0.rc1.591.g9c95f17f64-goog