KVM: selftests: document the default implementation of vm_vaddr_popul…

…ate_bitmap Explain the meaning of the bit manipulations of vm_vaddr_populate_bitmap. These correspond to the "canonical addresses" of x86 and other architectures, but that is not obvious. Signed-off-by: Paolo Bonzini <[email protected]>
wafgo · Dec 27, 2022 · feb84f6 · feb84f6
1 parent 2f5213b
commit feb84f6
Showing 1 changed file with 9 additions and 0 deletions.
diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
@@ -186,6 +186,15 @@ const struct vm_guest_mode_params vm_guest_mode_params[] = {
 _Static_assert(sizeof(vm_guest_mode_params)/sizeof(struct vm_guest_mode_params) == NUM_VM_MODES,
 	       "Missing new mode params?");
 
+/*
+ * Initializes vm->vpages_valid to match the canonical VA space of the
+ * architecture.
+ *
+ * The default implementation is valid for architectures which split the
+ * range addressed by a single page table into a low and high region
+ * based on the MSB of the VA. On architectures with this behavior
+ * the VA region spans [0, 2^(va_bits - 1)), [-(2^(va_bits - 1), -1].
+ */
 __weak void vm_vaddr_populate_bitmap(struct kvm_vm *vm)
 {
 	sparsebit_set_num(vm->vpages_valid,