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

Kai Huang
2021-02-01T00:34:17Z

On Sat, 30 Jan 2021 17:00:46 +0200 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?

Not 100% sure about your question. Assuming you are asking SGX hardware
behavior, SGX architecture adds a new modifier bit (27) to Exit Reason, similar
to new #PF.SGX bit.

Please refer to SDM Volume 3, Chapter 27.2.1 Basic VM-Exit Information.

Sean's commit msg already provides significant motivation of the change in this
patch.