linux-kernel - [PATCH v3 2/6] KVM: VMX: Avoid retpoline call for control register caused exits

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Date:   Wed,  1 Feb 2023 20:46:00 +0100
From:   Mathias Krause <minipli@...ecurity.net>
To:     kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Mathias Krause <minipli@...ecurity.net>
Subject: [PATCH v3 2/6] KVM: VMX: Avoid retpoline call for control register caused exits

Complement commit 4289d2728664 ("KVM: retpolines: x86: eliminate
retpoline from vmx.c exit handlers") and avoid a retpoline call for
control register accesses as well.

This speeds up guests that make heavy use of it, like grsecurity
kernels toggling CR0.WP to implement kernel W^X.

Signed-off-by: Mathias Krause <minipli@...ecurity.net>
---

Meanwhile I got my hands on a AMD system and while doing a similar change
for SVM gives a small measurable win (1.1% faster for grsecurity guests),
it would provide nothing for other guests, as the change I was testing was
specifically targeting CR0 caused exits.

A more general approach would instead cover CR3 and, maybe, CR4 as well.
However, that would require a lot more exit code compares, likely
vanishing the gains in the general case. So this tweak is VMX only.

 arch/x86/kvm/vmx/vmx.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index c788aa382611..c8198c8a9b55 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6538,6 +6538,8 @@ static int __vmx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t exit_fastpath)
 		return handle_external_interrupt(vcpu);
 	else if (exit_reason.basic == EXIT_REASON_HLT)
 		return kvm_emulate_halt(vcpu);
+	else if (exit_reason.basic == EXIT_REASON_CR_ACCESS)
+		return handle_cr(vcpu);
 	else if (exit_reason.basic == EXIT_REASON_EPT_MISCONFIG)
 		return handle_ept_misconfig(vcpu);
 #endif
-- 
2.39.1