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

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 1342d353 ("[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 195aefde ("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>

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;