linux-kernel - Re: [PATCH v3 10/21] KVM:x86: Add #CP support in guest exception classification

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Date:   Fri, 16 Jun 2023 11:57:47 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Weijiang Yang <weijiang.yang@...el.com>
Cc:     Chao Gao <chao.gao@...el.com>, pbonzini@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        peterz@...radead.org, rppt@...nel.org, binbin.wu@...ux.intel.com,
        rick.p.edgecombe@...el.com, john.allen@....com
Subject: Re: [PATCH v3 10/21] KVM:x86: Add #CP support in guest exception classification

On Fri, Jun 16, 2023, Weijiang Yang wrote:
> 
> On 6/16/2023 7:58 AM, Sean Christopherson wrote:
> > On Thu, Jun 08, 2023, Weijiang Yang wrote:
> > > On 6/6/2023 5:08 PM, Chao Gao wrote:
> > > > On Thu, May 11, 2023 at 12:08:46AM -0400, Yang Weijiang wrote:
> > > > > Add handling for Control Protection (#CP) exceptions(vector 21).
> > > > > The new vector is introduced for Intel's Control-Flow Enforcement
> > > > > Technology (CET) relevant violation cases.
> > > > > 
> > > > > Although #CP belongs contributory exception class, but the actual
> > > > > effect is conditional on CET being exposed to guest. If CET is not
> > > > > available to guest, #CP falls back to non-contributory and doesn't
> > > > > have an error code.
> > > > This sounds weird. is this the hardware behavior? If yes, could you
> > > > point us to where this behavior is documented?
> > > It's not SDM documented behavior.
> > The #CP behavior needs to be documented.  Please pester whoever you need to in
> > order to make that happen.
> 
> Do you mean documentation for #CP as an generic exception or the behavior in
> KVM as this patch shows?

As I pointed out two *years* ago, this entry in the SDM

  — The field's deliver-error-code bit (bit 11) is 1 if each of the following
    holds: (1) the interruption type is hardware exception; (2) bit 0
    (corresponding to CR0.PE) is set in the CR0 field in the guest-state area;
    (3) IA32_VMX_BASIC[56] is read as 0 (see Appendix A.1); and (4) the vector
    indicates one of the following exceptions: #DF (vector 8), #TS (10),
    #NP (11), #SS (12), #GP (13), #PF (14), or #AC (17).

needs to read something like

  — The field's deliver-error-code bit (bit 11) is 1 if each of the following
    holds: (1) the interruption type is hardware exception; (2) bit 0
    (corresponding to CR0.PE) is set in the CR0 field in the guest-state area;
    (3) IA32_VMX_BASIC[56] is read as 0 (see Appendix A.1); and (4) the vector
    indicates one of the following exceptions: #DF (vector 8), #TS (10),
    #NP (11), #SS (12), #GP (13), #PF (14), #AC (17), or #CP (21)[1]

    [1] #CP has an error code if and only if IA32_VMX_CR4_FIXED1 enumerates
        support for the 1-setting of CR4.CET.