kvm: vmx: Reduce size of vmcs_field_to_offset_table

The vmcs_field_to_offset_table was a rather sparse table of short
integers with a maximum index of 0x6c16, amounting to 55342 bytes. Now
that we are considering support for multiple VMCS12 formats, it would
be unfortunate to replicate that large, sparse table. Rotating the
field encoding (as a 16-bit integer) left by 6 reduces that table to
5926 bytes.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

arch/x86/kvm/vmx.c

@@ -686,10 +686,12 @@ static struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
 	return &(to_vmx(vcpu)->pi_desc);
 }
 
+#define ROL16(val, n) ((u16)(((u16)(val) << (n)) | ((u16)(val) >> (16 - (n)))))
 #define VMCS12_OFFSET(x) offsetof(struct vmcs12, x)
-#define FIELD(number, name)	[number] = VMCS12_OFFSET(name)
-#define FIELD64(number, name)	[number] = VMCS12_OFFSET(name), \
-				[number##_HIGH] = VMCS12_OFFSET(name)+4
+#define FIELD(number, name)	[ROL16(number, 6)] = VMCS12_OFFSET(name)
+#define FIELD64(number, name)						\
+	FIELD(number, name),						\
+	[ROL16(number##_HIGH, 6)] = VMCS12_OFFSET(name) + sizeof(u32)
 
 
 static u16 shadow_read_only_fields[] = {
@@ -908,9 +910,13 @@ static const unsigned short vmcs_field_to_offset_table[] = {
 
 static inline short vmcs_field_to_offset(unsigned long field)
 {
-	BUILD_BUG_ON(ARRAY_SIZE(vmcs_field_to_offset_table) > SHRT_MAX);
+	unsigned index;
+
+	if (field >> 15)
+		return -ENOENT;
 
-	if (field >= ARRAY_SIZE(vmcs_field_to_offset_table))
+	index = ROL16(field, 6);
+	if (index >= ARRAY_SIZE(vmcs_field_to_offset_table))
 		return -ENOENT;
 
 	/*
@@ -919,10 +925,10 @@ static inline short vmcs_field_to_offset(unsigned long field)
 	 */
 	asm("lfence");
 
-	if (vmcs_field_to_offset_table[field] == 0)
+	if (vmcs_field_to_offset_table[index] == 0)
 		return -ENOENT;
 
-	return vmcs_field_to_offset_table[field];
+	return vmcs_field_to_offset_table[index];
 }
 
 static inline struct vmcs12 *get_vmcs12(struct kvm_vcpu *vcpu)