KVM: x86: Unload MMU on guest TLB flush if TDP disabled to force MMU sync

When using shadow paging, unload the guest MMU when emulating a guest TLB
flush to ensure all roots are synchronized.  From the guest's perspective,
flushing the TLB ensures any and all modifications to its PTEs will be
recognized by the CPU.

Note, unloading the MMU is overkill, but is done to mirror KVM's existing
handling of INVPCID(all) and ensure the bug is squashed.  Future cleanup
can be done to more precisely synchronize roots when servicing a guest
TLB flush.

If TDP is enabled, synchronizing the MMU is unnecessary even if nested
TDP is in play, as a "legacy" TLB flush from L1 does not invalidate L1's
TDP mappings.  For EPT, an explicit INVEPT is required to invalidate
guest-physical mappings; for NPT, guest mappings are always tagged with
an ASID and thus can only be invalidated via the VMCB's ASID control.

This bug has existed since the introduction of KVM_VCPU_FLUSH_TLB.
It was only recently exposed after Linux guests stopped flushing the
local CPU's TLB prior to flushing remote TLBs (see commit 4ce94ea,
"x86/mm/tlb: Flush remote and local TLBs concurrently"), but is also
visible in Windows 10 guests.

Tested-by: Maxim Levitsky <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Fixes: f38a7b7 ("KVM: X86: support paravirtualized help for TLB shootdowns")
Signed-off-by: Lai Jiangshan <[email protected]>
[sean: massaged comment and changelog]
Message-Id: <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Cc: [email protected]
Signed-off-by: Paolo Bonzini <[email protected]>

Author: Lai Jiangshan

arch/x86/kvm/x86.c

@@ -3072,6 +3072,19 @@ static void kvm_vcpu_flush_tlb_all(struct kvm_vcpu *vcpu)
 static void kvm_vcpu_flush_tlb_guest(struct kvm_vcpu *vcpu)
 {
 	++vcpu->stat.tlb_flush;
+
+	if (!tdp_enabled) {
+               /*
+		 * A TLB flush on behalf of the guest is equivalent to
+		 * INVPCID(all), toggling CR4.PGE, etc., which requires
+		 * a forced sync of the shadow page tables.  Unload the
+		 * entire MMU here and the subsequent load will sync the
+		 * shadow page tables, and also flush the TLB.
+		 */
+		kvm_mmu_unload(vcpu);
+		return;
+	}
+
 	static_call(kvm_x86_tlb_flush_guest)(vcpu);
 }