From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793831810; b=4pfItg9f6X1m8Hy5hqUUyEK5CZ0HduKTnMfckdTdzT26xM2q+pneeJFr+6BA/K6NS0XJ F1n0YzHu7j3D0srdK8bKp/6uC7fw+I3eC7zpGhyxqMH/bGWYLnHx9vOhi3/pZuOToDsAK dOVOzOXG+acR+Yj88yF0XEa5We0287j+H8+ia0CS6qDEynBpX0PFH+UH77fWb3kUi/t5L zm4iZUG7GRRSnX6EDC7jC9hLg9TQVTmp7dt/apnK2agQ50ck0D34vbwz+hNbQ/Kgu/cch N2kqKdd0B1ogATUgFRI2Mn74K19GEOMDN1LZVGMeJIaNADjcC+swmwVS5q2Kalf/VWw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793831810; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:User-Agent:X-stable:X-Patchwork-Hint: MIME-Version:Content-Transfer-Encoding; bh=iJ+B+lRT15iaj5dOaUM14RuspvgUFx4l/miAKyRASUw=; b=pTmyOe9kykZ907sAm81KIP44JbbOZBcFRJzxfbYdnCbbXBY4ZXnW5Q/NMTk4h8jm+Usf Byk14K4K5yX3q1ThfZbFV+ZGWaWqJeQ3vGFLxrXb7I2h8SIrD+ud/vahFu2mNt80BSAwD aH1u1QCyAGq59v7oRYXOQQnlpWEZY9oLjH59HG58P+R87o6oi6qm4Ut1d+wCT5e8oMFPJ TmgU253OP8hX9ntIQ0HCPzLQZexi/+L+9Oq+M3y7fbsOWaPORWxfcHyh446IOSjvdXYiq ptsRmmtQzTGz2enycoLtHxLFYJoOg0zhfFsMhZ6D7HVGG/2QBomgxvMzdv/xSyVCNrQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=kqUyQ1no; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 31666606A860; Wed, 4 Nov 2026 22:36:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=kqUyQ1no Received: by smtp.kernel.org (Postfix) with ESMTPSA id B83801F00919; Wed, 4 Nov 2026 22:36:49 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1793831810; bh=iJ+B+lRT15iaj5dOaUM14RuspvgUFx4l/miAKyRASUw=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=kqUyQ1noAh5YUG+usWe6AIbnKqTI84znd610CS76Y1hnEY/W1UjYyhJKS8xnjzeb5 slL0qNbW2rCqFWztaKrcSyih0cTdYVVgRoH0bEspRBBEwptpZ/FqNmPDOx9PCFKGrh s1/quG73jPhWkdHqCcBTG+Uq18cReSwh3YW1tK6w= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Stefan Teodorescu , Yosry Ahmed , Nikunj A Dadhania , Sean Christopherson Subject: [PATCH 6.18 0925/1005] KVM: SVM: Trigger new ASID allocation in both VMCBs on pCPU switch Date: Wed, 4 Nov 2026 16:10:58 -0500 Message-ID: <20261104211109.051255670@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261104211043.516122098@linuxfoundation.org> References: <20261104211043.516122098@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore X-Mailing-List: patches@lists.linux.dev List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Yosry Ahmed commit bf260dc9e7b8bfcd06c1a8e739797b1cf0917c72 upstream. When the pCPU where the VMCB was most 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 tracked pCPU for vmcb02 to -1 on nested enablement as hardening, so that a new ASID allocation is always triggered when nested is disabled and re-enabled. 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@linux.dev [2] https://lore.kernel.org/kvm/20260728003557.1136583-1-yosry@kernel.org Cc: stable@vger.kernel.org Reported-by: Stefan Teodorescu Signed-off-by: Yosry Ahmed Reviewed-by: Nikunj A Dadhania Link: https://patch.msgid.link/20260903223258.486034-1-yosry@kernel.org Signed-off-by: Sean Christopherson Signed-off-by: Greg Kroah-Hartman --- arch/x86/kvm/svm/nested.c | 1 + arch/x86/kvm/svm/svm.c | 19 +++++++++++++++---- 2 files changed, 16 insertions(+), 4 deletions(-) --- a/arch/x86/kvm/svm/nested.c +++ b/arch/x86/kvm/svm/nested.c @@ -1346,6 +1346,7 @@ int svm_allocate_nested(struct vcpu_svm if (!svm->nested.msrpm) goto err_free_vmcb02; + svm->nested.vmcb02.cpu = -1; svm->nested.initialized = true; return 0; --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3613,14 +3613,25 @@ static int pre_svm_run(struct kvm_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; + svm->nested.vmcb02.asid_generation = 0; } if (sev_guest(vcpu->kvm))