Re: [RFC PATCH v3 16/27] KVM: VMX: Convert vcpu_vmx.exit_reason to a union

Sean Christopherson
2021-02-01T17:13:49Z

On Sat, Jan 30, 2021, Jarkko Sakkinen wrote:
> On Tue, Jan 26, 2021 at 10:31:37PM +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.
>
> I *believe* that the change is correct but I dropped in the last paragraph
> - most likely only because of lack of expertise in this area.
>
> I ask the most basic question: why SGX will add new modifier bits?

Register state is loaded with synthetic state and/or trampoline state on VM-Exit
from enclaves. For all intents and purposes, emulation and other VMM/hypervisor
behavior that accesses vCPU state is impossible. E.g. on a #UD VM-Exit, RIP
will point at the AEP; emulating would emulate some random instruction in the
untrusted runtime, not the instruction that faulted.

Hardware sets the "exit from enclave" modifier flag so that the VMM can try and
do something moderately sane, e.g. inject a #UD into the guest instead of
attempting to emulate random instructions.