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From:   Vitaly Kuznetsov <vkuznets@redhat.com>
To:     Wanpeng Li <kernellwp@gmail.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Subject: Re: [PATCH 1/2] KVM: X86: Single target IPI fastpath
In-Reply-To: <1573283135-5502-1-git-send-email-wanpengli@tencent.com>
References: <1573283135-5502-1-git-send-email-wanpengli@tencent.com>
Date:   Mon, 11 Nov 2019 14:06:52 +0100
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Wanpeng Li <kernellwp@gmail.com> writes:

> +
>  static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  {
>  =09struct vcpu_vmx *vmx =3D to_vmx(vcpu);
> @@ -6615,6 +6645,12 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  =09=09=09=09  | (1 << VCPU_EXREG_CR3));
>  =09vcpu->arch.regs_dirty =3D 0;
> =20
> +=09vmx->exit_reason =3D vmx->fail ? 0xdead : vmcs_read32(VM_EXIT_REASON)=
;
> +=09vcpu->fast_vmexit =3D false;
> +=09if (!is_guest_mode(vcpu) &&
> +=09=09vmx->exit_reason =3D=3D EXIT_REASON_MSR_WRITE)
> +=09=09vcpu->fast_vmexit =3D handle_ipi_fastpath(vcpu);

I have to admit this looks too much to me :-( Yes, I see the benefits of
taking a shortcut (by actualy penalizing all other MSR writes) but the
question I have is: where do we stop?

Also, this 'shortcut' creates an imbalance in tracing: you don't go down
to kvm_emulate_wrmsr() so handle_ipi_fastpath() should probably gain a
tracepoint.

Looking at 'fast_vmexit' name makes me think this is something
generic. Is this so? Maybe we can create some sort of an infrastructure
for fast vmexit handling and make it easy to hook things up to it?

(Maybe it's just me who thinks the codebase is already too complex,
let's see what Paolo and other reviewers have to say).

--=20
Vitaly