linux-kernel - Re: [PATCH] KVM: x86/mmu: Avoid retpoline on ->page

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Date:   Wed, 12 Feb 2020 12:55:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Avoid retpoline on ->page_fault() with TDP

On 07/02/20 16:55, Sean Christopherson wrote:
> It becomes a matter of weighing the maintenance cost and robustness against
> the performance benefits.  For the TDP case, amost no one (that cares about
> performance) uses shadow paging, the change is very explicit, tiny and
> isolated, and TDP page fault are a hot path, e.g. when booting the VM.
> I.e. low maintenance overhead, still robust, and IMO worth the shenanigans.

The "NULL" trick does not seem needed though.  Any objections to this?

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index 9277ee8a54a5..a647601c9e1c 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -109,7 +109,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 					u32 err, bool prefault)
 {
 #ifdef CONFIG_RETPOLINE
-	if (likely(!vcpu->arch.mmu->page_fault))
+	if (likely(vcpu->arch.mmu->page_fault == kvm_tdp_page_fault))
 		return kvm_tdp_page_fault(vcpu, cr2_or_gpa, err, prefault);
 #endif
 	return vcpu->arch.mmu->page_fault(vcpu, cr2_or_gpa, err, prefault);
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 5267f1440677..87e9ba27ada1 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4925,12 +4925,7 @@ static void init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
 		return;
 
 	context->mmu_role.as_u64 = new_role.as_u64;
-#ifdef CONFIG_RETPOLINE
-	/* Nullify ->page_fault() to use direct kvm_tdp_page_fault() call. */
-	context->page_fault = NULL;
-#else
 	context->page_fault = kvm_tdp_page_fault;
-#endif
 	context->sync_page = nonpaging_sync_page;
 	context->invlpg = nonpaging_invlpg;
 	context->update_pte = nonpaging_update_pte;

Paolo