[RFC PATCH v2 06/25] KVM: SVM: Fallback to 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 vCPU (like VMX),
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. The only other
current caller is from SEV code, and no CPUs support SEV but not
FLUSHBYASID (at least according to AI).

Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---
 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 a7cc3cb101e57..159abf841738f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4185,10 +4185,7 @@ 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 5b4613b4d7bc2..4bf8afdc77cbd 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -478,7 +478,10 @@ static inline void vmcb_set_gpat(struct vmcb *vmcb, u64 data)
 
 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)
-- 
2.54.0.1136.gdb2ca164c4-goog