Re: [RFC PATCH v2 15/26] KVM: VMX: Convert vcpu_vmx.exit_reason to a union

Jarkko Sakkinen
2021-01-20T15:38:13Z

On Mon, Jan 18, 2021 at 04:28:26PM +1300, Kai Huang wrote:
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> Convert vcpu_vmx.exit_reason from a u32 to a union (of size u32). The
> full VM_EXIT_REASON field is comprised of a 16-bit basic exit reason in
> bits 15:0, and single-bit modifiers in bits 31:16.
>
> Historically, KVM has only had to worry about handling the "failed
> VM-Entry" modifier, which could only be set in very specific flows and
> required dedicated handling. I.e. manually stripping the FAILED_VMENTRY
> bit was a somewhat viable approach. But even with only a single bit to
> worry about, KVM has had several bugs related to comparing a basic exit
> reason against the full exit reason store in vcpu_vmx.
>
> Upcoming Intel features, e.g. SGX, will add new modifier bits that can
> be set on more or less any VM-Exit, as opposed to the significantly more
> restricted FAILED_VMENTRY, i.e. correctly handling everything in one-off
> flows isn't scalable. Tracking exit reason in a union forces code to
> explicitly choose between consuming the full exit reason and the basic
> exit, and is a convenient way to document and access the modifiers.
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Signed-off-by: Kai Huang <kai.huang@intel.com>
> ---
> arch/x86/kvm/vmx/nested.c | 42 +++++++++++++++---------
> arch/x86/kvm/vmx/vmx.c | 68 ++++++++++++++++++++-------------------
> arch/x86/kvm/vmx/vmx.h | 25 +++++++++++++-
> 3 files changed, 86 insertions(+), 49 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 0fbb46990dfc..f112c2482887 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3311,7 +3311,11 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
> struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> enum vm_entry_failure_code entry_failure_code;
> bool evaluate_pending_interrupts;
> - u32 exit_reason, failed_index;
> + u32 failed_index;
> + union vmx_exit_reason exit_reason = {
> + .basic = -1,
> + .failed_vmentry = 1,
> + };

Instead, put this declaration to the correct place, following the
reverse christmas tree ordering:

union vmx_exit_reason exit_reason = {};

And after declarations:

exit_reason.basic = -1;
exit_reason.failed_vmentry = 1;

More pleasing for the eye.

/Jarkko