linux-kernel - Re: [PATCH 02/11] KVM: VMX: Handle APIC-write offset wrangling in VMX code

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Message-ID: <YgvVNNvx4NGDSGUA@google.com>
Date:   Tue, 15 Feb 2022 16:30:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Gao <chao.gao@...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>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Zeng Guang <guang.zeng@...el.com>,
        Maxim Levitsky <mlevitsk@...hat.com>
Subject: Re: [PATCH 02/11] KVM: VMX: Handle APIC-write offset wrangling in
 VMX code

On Tue, Feb 15, 2022, Chao Gao wrote:
> >--- a/arch/x86/kvm/vmx/vmx.c
> >+++ b/arch/x86/kvm/vmx/vmx.c
> >@@ -5302,9 +5302,16 @@ static int handle_apic_eoi_induced(struct kvm_vcpu *vcpu)
> > static int handle_apic_write(struct kvm_vcpu *vcpu)
> > {
> > 	unsigned long exit_qualification = vmx_get_exit_qual(vcpu);
> >-	u32 offset = exit_qualification & 0xfff;
> > 
> >-	/* APIC-write VM exit is trap-like and thus no need to adjust IP */
> >+	/*
> >+	 * APIC-write VM-Exit is trap-like, KVM doesn't need to advance RIP and
> >+	 * hardware has done any necessary aliasing, offset adjustments, etc...
> >+	 * for the access.  I.e. the correct value has already been  written to
> >+	 * the vAPIC page for the correct 16-byte chunk.  KVM needs only to
> >+	 * retrieve the register value and emulate the access.
> >+	 */
> >+	u32 offset = exit_qualification & 0xff0;
> 
> Can we take this opportunity to remove offset/exit_qualification?
> They are used just once.

Definitely should have dropped exit_qualification, not sure why I didn't.

I'd prefer to keep offset to document what is held in vmcs.EXIT_QUALIFICATION
without having to add an explicit comment.