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

Kai Huang
2021-02-02T19:26:33Z

On Tue, 2 Feb 2021 19:24:42 +0200 Jarkko Sakkinen wrote:
> On Mon, Feb 01, 2021 at 01:32:59PM +1300, Kai Huang wrote:
> > 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
>
> BTW, SGX is not an upcoming CPU feature.

Probably Sean was implying: "Upcoming CPU features that will be supported by
Linux". I don't see big deal here.

>
> Also, broadly speaking of upcoming features is not right thing to do.
> Better just to scope this down SGX. Theoretically upcoming CPU features
> can do pretty much anything. This is change is first and foremost done
> for SGX.
>
> > > > 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.
>
> Just describe why SGX requires this. That's all.

This patch is to change vmexit info from u32 to union, because at least one
additional modifier is going to be added, due to SGX. So the motivation of this
patch is the fact that "one or more additional modifier bits will be added",
and SGX is just example.

So I don't think adding too much SGX backgroud in *THIS* patch is needed.
And another patch:

[RFC PATCH v3 21/27] KVM: VMX: Add basic handling of VM-Exit from SGX enclave

already has enough information of "why new modifier bit is aadded for SGX".
Sean also replied to you.

Please look at that patch and see whether it satisfies you.

>
> /Jarkko