[RFC PATCH 04/24] KVM: SVM: Flush everything if FLUSHBYASID is not available

[Yosry Ahmed]

Currently, if FLUSHBYASID is not available when performing a TLB flush,
the fallback is decrementing the ASID generation to trigger allocating a
new ASID. In preparation for using a static ASID per VM, just fallback
to flushing everything if FLUSHBYASID is not available. This is probably
worse from a performance perspective, but FLUSHBYASID has been around
for ~15 years and it's not worth carrying the complexity.

The fallback logic is moved within vmcb_set_flush_asid(), as more
callers will be added and will need the fallback as well.

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
---
 arch/x86/kvm/svm/svm.c | 5 +----
 arch/x86/kvm/svm/svm.h | 5 ++++-
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0e302ae9a8435..5f71b125010d9 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4005,10 +4005,7 @@ static void svm_flush_tlb_asid(struct kvm_vcpu *vcpu)
 	 * unconditionally does a TLB flush on both nested VM-Enter and nested
 	 * VM-Exit (via kvm_mmu_reset_context()).
 	 */
-	if (static_cpu_has(X86_FEATURE_FLUSHBYASID))
-		vmcb_set_flush_asid(svm->vmcb);
-	else
-		svm->current_vmcb->asid_generation--;
+	vmcb_set_flush_asid(svm->vmcb);
 }
 
 static void svm_flush_tlb_current(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index d2c49cbfbf1ca..843a29a6d150e 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -416,7 +416,10 @@ static inline bool vmcb_is_dirty(struct vmcb *vmcb, int bit)
 
 static inline void vmcb_set_flush_asid(struct vmcb *vmcb)
 {
-	vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
+	if (static_cpu_has(X86_FEATURE_FLUSHBYASID))
+		vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
+	else
+		vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ALL_ASID;
 }
 
 static inline void vmcb_clr_flush_asid(struct vmcb *vmcb)
-- 
2.49.0.395.g12beb8f557-goog