linux-kernel - Re: [RESEND v4 1/2] KVM: VMX: Convert vcpu_vmx.exit

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Date:   Tue, 20 Oct 2020 15:01:58 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Chenyi Qiang <chenyi.qiang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RESEND v4 1/2] KVM: VMX: Convert vcpu_vmx.exit_reason to a union

On Mon, Oct 12, 2020 at 11:35:41AM +0800, Chenyi Qiang wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.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.
> 
> No functional change intended.
> 
> Cc: Xiaoyao Li <xiaoyao.li@...el.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>

This needs your SOB since you are involved in the handling of the patch.