KVM: x86: Simplify retrieving the page offset when loading PDTPRs

sean-jc · pull[bot] · commit 5591310cac99 · 2021-11-02T22:55:28.000Z
Replace impressively complex "logic" for computing the page offset from CR3 when loading PDPTRs. Unlike other paging modes, the address held in CR3 for PAE paging is 32-byte aligned, i.e. occupies bits 31:5, thus bits 11:5 need to be used as the offset from the gfn when reading PDPTRs. The existing calculation originated in commit 1342d35 ("[PATCH] KVM: MMU: Load the pae pdptrs on cr3 change like the processor does"), which read the PDPTRs from guest memory as individual 8-byte loads. At the time, the so called "offset" was the base index of PDPTR0 as a _u64_, not a byte offset. Naming aside, the computation was useful and arguably simplified the overall flow. Unfortunately, when commit 195aefd ("KVM: Add general accessors to read and write guest memory") added accessors with offsets at byte granularity, the cleverness of the original code was lost and KVM was left with convoluted code for a simple operation. No functional change intended. Signed-off-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-Id: <20210831164224.1119728-4-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
@@ -801,7 +801,6 @@ static inline u64 pdptr_rsvd_bits(struct kvm_vcpu *vcpu)
 int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3)
 {
 	gfn_t pdpt_gfn = cr3 >> PAGE_SHIFT;
-	unsigned offset = (((cr3 & (PAGE_SIZE-1)) >> 5) << 2) * sizeof(u64);
 	gpa_t real_gpa;
 	int i;
 	int ret;
@@ -816,8 +815,9 @@ int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3)
 	if (real_gpa == UNMAPPED_GVA)
 		return 0;
 
+	/* Note the offset, PDPTRs are 32 byte aligned when using PAE paging. */
 	ret = kvm_vcpu_read_guest_page(vcpu, gpa_to_gfn(real_gpa), pdpte,
-				       offset, sizeof(pdpte));
+				       cr3 & GENMASK(11, 5), sizeof(pdpte));
 	if (ret < 0)
 		return 0;
 
