[PATCH] KVM: SVM: Trigger new ASID allocation in both VMCBs on pCPU switch

[Yosry Ahmed]

When the pCPU where the VMCB was mostly recently used is switched, reset
the ASID generation in both VMCBs, triggering new ASID allocation for
the immediate VMRUN as well as the next VMRUN on the other VMCB.

The ASID is shared between vmcb01 and vmcb02, and gets flushed on every
nested transition. However, since pCPU tracking is done per VMCB, it is
possible for one VMCB to allocate a new ASID when migrated to a new
pCPU, and then the other VMCB reuses that ASID on the old pCPU. This can
result in the same ASID being used by multiple vCPUs on the old pCPU.

Example scenario:
- vCPU runs on pCPU A, vmcb01 is active, asid=1.
- vCPU migrates to pCPU B, vmcb01 pCPU changes, new asid=2.
- Another vCPU runs on pCPU A and allocates asid=2 as well.
- vCPU migrates back to pCPU A, and then switches to vmcb02 before it
  runs again with vmcb01.
- No pCPU switch is detected for vmcb02, so VMRUN is done with asid=2.
- Two vCPUs end up using asid=2 on the same pCPU.

Keep the VMCB dirtying to the active VMCB only. Clean bits are tracked
by a pCPU for each VMCB, so do not unnecessarily dirty a VMCB if its
pCPU does not change.

Additionally, initialize the tracker pCPU for vmcb02 to -1 on nested
enablement, so that the new ASID allocation in the scenario above
happens even if EFER.SVME is disabled in L1 before migrating to the new
pCPU (so asid_generation in vmcb02 is not reset), but enabled before
returning to the old pCPU.

No performance regression was noticed when overcommitting L1 vCPUs in L0
(to force rescheduling), pinning L1 <-> L2 vCPUs, and running CPUID in a
tight loop bouncing between 2 vCPUs in L2.

An alternative (and perhaps more proper) fix would be tracking the ASID
per-VMCB instead (e.g. [1]). However, that's a more involved change, and
it would result in having different ASIDs for L1 and L2 without actually
properly maintaining them. It would probably work because all TLB
flushes target the current VMCB, and the other VMCB is always flushed on
nested transitions, but the code ends up in an arguably more fragile
state. Punt a proper clean fix to an incoming (and overdue) overhaul of
SVM's ASID usage [2].

[1]https://lore.kernel.org/lkml/20250205182402.2147495-2-yosry.ahmed@xxxxxxxxx/
[2]https://lore.kernel.org/kvm/20260728003557.1136583-1-yosry@xxxxxxxxxx/

Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Stefan Teodorescu <fane@xxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
---

I wasn't sure if the last paragraph (or parts of it) fit in the
changelog or below ---, so I just put it all in the changelog, but feel
free to move things around.

---
 arch/x86/kvm/svm/nested.c |  1 +
 arch/x86/kvm/svm/svm.c    | 20 ++++++++++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
index 73f37b050d0a0..0c55c71fc6010 100644
--- a/arch/x86/kvm/svm/nested.c
+++ b/arch/x86/kvm/svm/nested.c
@@ -1494,6 +1494,7 @@ int svm_allocate_nested(struct vcpu_svm *svm)
 	if (!svm->nested.msrpm)
 		goto err_free_vmcb02;
 
+	svm->nested.vmcb02.cpu = -1;
 	svm->nested.initialized = true;
 	return 0;
 
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index ea647938a2a65..c388c6598463e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3765,14 +3765,26 @@ static int pre_svm_run(struct kvm_vcpu *vcpu)
 	struct vcpu_svm *svm = to_svm(vcpu);
 
 	/*
-	 * If the previous vmrun of the vmcb occurred on a different physical
-	 * cpu, then mark the vmcb dirty and assign a new asid.  Hardware's
-	 * vmcb clean bits are per logical CPU, as are KVM's asid assignments.
+	 * If the previous VMRUN of the VMCB occurred on a different physical
+	 * cpu, then mark the VMCB dirty as hardware's clean bits are per pCPU.
+	 *
+	 * Reset the ASID generation in both VMCBs. This will lead to assigning
+	 * a new ASID now, and then again when switching to the other VMCB.
+	 * However, this is needed as the ASID is shared between the VMCBs, and
+	 * otherwise it would be possible to use an ASID allocated on one pCPU
+	 * on another, for example:
+	 * - vCPU migrates from pCPU A to pCPU B, allocates a new ASID.
+	 * - vCPU migrates back to pCPU A, and then switches the VMCB.
+	 * - The new VMCB does not detect a pCPU change and runs on pCPU A with
+	 *   the new ASID allocated on pCPU B, which is potentially used by
+	 *   another vCPU/VM.
 	 */
 	if (unlikely(svm->current_vmcb->cpu != vcpu->cpu)) {
-		svm->current_vmcb->asid_generation = 0;
 		vmcb_mark_all_dirty(svm->vmcb);
 		svm->current_vmcb->cpu = vcpu->cpu;
+		svm->vmcb01.asid_generation = 0;
+		if (svm->nested.initialized)
+			svm->nested.vmcb02.asid_generation = 0;
         }
 
 	if (is_sev_guest(vcpu))
-- 
2.55.0.979.g7e5102b832-goog