From cb9fb5fc12ef8a7c5129c7db0f80df45726b8dcd Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Thu, 13 Jun 2024 12:01:03 -0700 Subject: [PATCH] KVM: nVMX: Update VMCS12_REVISION comment to state it should never change Rewrite the comment above VMCS12_REVISION to unequivocally state that the ID must never change. KVM_{G,S}ET_NESTED_STATE have been officially supported for some time now, i.e. changing VMCS12_REVISION would break userspace. Opportunistically add a blurb to the CHECK_OFFSET() comment to make it explicitly clear that new fields are allowed, i.e. that the restriction on the layout is all about backwards compatibility. No functional change intended. Cc: Jim Mattson Reviewed-by: Jim Mattson Link: https://lore.kernel.org/r/20240613190103.1054877-1-seanjc@google.com Signed-off-by: Sean Christopherson --- arch/x86/kvm/vmx/vmcs12.h | 14 ++++++++------ 1 file changed, 8 insertions(+), 6 deletions(-) diff --git a/arch/x86/kvm/vmx/vmcs12.h b/arch/x86/kvm/vmx/vmcs12.h index 01936013428b5..56fd150a6f243 100644 --- a/arch/x86/kvm/vmx/vmcs12.h +++ b/arch/x86/kvm/vmx/vmcs12.h @@ -188,12 +188,13 @@ struct __packed vmcs12 { }; /* - * VMCS12_REVISION is an arbitrary id that should be changed if the content or - * layout of struct vmcs12 is changed. MSR_IA32_VMX_BASIC returns this id, and - * VMPTRLD verifies that the VMCS region that L1 is loading contains this id. + * VMCS12_REVISION is KVM's arbitrary ID for the layout of struct vmcs12. KVM + * enumerates this value to L1 via MSR_IA32_VMX_BASIC, and checks the revision + * ID during nested VMPTRLD to verify that L1 is loading a VMCS that adhere's + * to KVM's virtual CPU definition. * - * IMPORTANT: Changing this value will break save/restore compatibility with - * older kvm releases. + * DO NOT change this value, as it will break save/restore compatibility with + * older KVM releases. */ #define VMCS12_REVISION 0x11e57ed0 @@ -206,7 +207,8 @@ struct __packed vmcs12 { #define VMCS12_SIZE KVM_STATE_NESTED_VMX_VMCS_SIZE /* - * For save/restore compatibility, the vmcs12 field offsets must not change. + * For save/restore compatibility, the vmcs12 field offsets must not change, + * although appending fields and/or filling gaps is obviously allowed. */ #define CHECK_OFFSET(field, loc) \ ASSERT_STRUCT_OFFSET(struct vmcs12, field, loc) -- 2.47.2