linux-kernel - [PATCH] KVM/x86: add comment to kvm_mmu_do_page

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Message-ID: <20241108161416.28552-1-jgross@suse.com>
Date: Fri,  8 Nov 2024 17:14:16 +0100
From: Juergen Gross <jgross@...e.com>
To: linux-kernel@...r.kernel.org,
	x86@...nel.org,
	kvm@...r.kernel.org
Cc: Juergen Gross <jgross@...e.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH] KVM/x86: add comment to kvm_mmu_do_page_fault()

On a first glance it isn't obvious why calling kvm_tdp_page_fault() in
kvm_mmu_do_page_fault() is special cased, as the general case of using
an indirect case would result in calling of kvm_tdp_page_fault()
anyway.

Add a comment to explain the reason.

Signed-off-by: Juergen Gross <jgross@...e.com>
---
 arch/x86/kvm/mmu/mmu_internal.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
index c98827840e07..a49cd0c438a1 100644
--- a/arch/x86/kvm/mmu/mmu_internal.h
+++ b/arch/x86/kvm/mmu/mmu_internal.h
@@ -322,6 +322,10 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
 		fault.slot = kvm_vcpu_gfn_to_memslot(vcpu, fault.gfn);
 	}
 
+	/*
+	 * With retpoline being active an indirect call is rather expensive,
+	 * so do a direct call in the most common case.
+	 */
 	if (IS_ENABLED(CONFIG_MITIGATION_RETPOLINE) && fault.is_tdp)
 		r = kvm_tdp_page_fault(vcpu, &fault);
 	else
-- 
2.43.0