[PATCH v3 32/37] KVM: x86/mmu: Add module param to force TLB flush on root reuse

[Sean Christopherson]

Add a module param, flush_on_reuse, to override skip_tlb_flush and
skip_mmu_sync 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 and/or MMU
sync bugs are exposed/introduced by upcoming changes to stop
unconditionally flushing on nested VMX transitions.

Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
Suggested-by: Junaid Shahid <junaids@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 arch/x86/kvm/mmu/mmu.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 06e94ca59a2d..6a986b66c867 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_flush_and_sync_on_reuse;
+module_param_named(flush_on_reuse, force_flush_and_sync_on_reuse, bool, 0644);
+
 /*
  * When setting this variable to true it enables Two-Dimensional-Paging
  * where the hardware walks 2 page tables:
@@ -4322,9 +4325,9 @@ static void __kvm_mmu_new_cr3(struct kvm_vcpu *vcpu, gpa_t new_cr3,
 	 */
 	kvm_make_request(KVM_REQ_LOAD_MMU_PGD, vcpu);
 
-	if (!skip_mmu_sync)
+	if (!skip_mmu_sync || force_flush_and_sync_on_reuse)
 		kvm_make_request(KVM_REQ_MMU_SYNC, vcpu);
-	if (!skip_tlb_flush)
+	if (!skip_tlb_flush || force_flush_and_sync_on_reuse)
 		kvm_make_request(KVM_REQ_TLB_FLUSH_CURRENT, vcpu);
 
 	/*
-- 
2.24.1