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@xxxxxxxxx> > > > > 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.
