linux-kernel - [PATCH v2 30/32] KVM: x86/mmu: Add module param to force TLB flush on root reuse

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Date:   Mon, 16 Mar 2020 21:52:36 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.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, Ben Gardon <bgardon@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Liran Alon <liran.alon@...cle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        John Haxby <john.haxby@...cle.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH v2 30/32] KVM: x86/mmu: Add module param to force TLB flush on root reuse

Add a module param, flush_on_reuse, to override skip_tlb_flush when
performing a so called "fast cr3 switch", i.e. when reusing a cached
root.  The primary motiviation for the control is to provide a fallback
mechanism in the event that TLB flushing bugs are exposed/introduced by
upcoming changes to stop unconditionally flushing on nested VMX
transitions.

Suggested-by: Jim Mattson <jmattson@...gle.com>
Suggested-by: Junaid Shahid <junaids@...gle.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/mmu/mmu.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 97d906a42e81..b98482b60748 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -78,6 +78,9 @@ module_param_cb(nx_huge_pages_recovery_ratio, &nx_huge_pages_recovery_ratio_ops,
 		&nx_huge_pages_recovery_ratio, 0644);
 __MODULE_PARM_TYPE(nx_huge_pages_recovery_ratio, "uint");
 
+static bool __read_mostly force_tlb_flush_on_reuse;
+module_param_named(flush_on_reuse, force_tlb_flush_on_reuse, bool, 0644);
+
 /*
  * When setting this variable to true it enables Two-Dimensional-Paging
  * where the hardware walks 2 page tables:
@@ -4340,6 +4343,9 @@ static void __kvm_mmu_new_cr3(struct kvm_vcpu *vcpu, gpa_t new_cr3,
 			      union kvm_mmu_page_role new_role,
 			      bool skip_tlb_flush)
 {
+	if (force_tlb_flush_on_reuse)
+		skip_tlb_flush = false;
+
 	if (!fast_cr3_switch(vcpu, new_cr3, new_role, skip_tlb_flush))
 		kvm_mmu_free_roots(vcpu, vcpu->arch.mmu,
 				   KVM_MMU_ROOT_CURRENT);
-- 
2.24.1