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Tue, 16 Jun 2026 16:34:20 +0000 (GMT) Received: from smtpav03.fra02v.mail.ibm.com (smtpav03.fra02v.mail.ibm.com [10.20.54.102]) by smtprelay01.fra02v.mail.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 65GGYGpa62914894 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 16 Jun 2026 16:34:16 GMT Received: from smtpav03.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C590620043; Tue, 16 Jun 2026 16:34:16 +0000 (GMT) Received: from smtpav03.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C760420040; Tue, 16 Jun 2026 16:34:12 +0000 (GMT) Received: from localhost.localdomain (unknown [9.39.31.150]) by smtpav03.fra02v.mail.ibm.com (Postfix) with ESMTP; Tue, 16 Jun 2026 16:34:12 +0000 (GMT) From: Amit Machhiwal To: linuxppc-dev@lists.ozlabs.org, Madhavan Srinivasan Cc: Amit Machhiwal , Vaibhav Jain , Harsh Prateek Bora , Ritesh Harjani , Anushree Mathur , Gautam Menghani , Mukesh Kumar Chaurasiya , Nicholas Piggin , Michael Ellerman , "Christophe Leroy (CS GROUP)" , Thomas Huth , kvm@vger.kernel.org, stable@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v4] KVM: PPC: Book3S HV: Validate arch_compat against host compatibility mode Date: Tue, 16 Jun 2026 22:04:05 +0530 Message-ID: <20260616163405.96962-1-amachhiw@linux.ibm.com> X-Mailer: git-send-email 2.50.1 X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Info: AW1haW4tMjYwNjE2MDE2NyBTYWx0ZWRfX9cNhnHOqh7as QhkJVNQTKDfW9r963Q1udV+w6U+JeLEtlm0CgoVBWdJxziD1DHh53UpMeqYkm5nveLdOFuLY1cx hnewYp3QC8ZXRDh9vALMgREKT81+Sg0= X-Proofpoint-ORIG-GUID: FIihs5FllMLPCxPalCaX9-QVcCx5oX0s X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNjE2MDE2NyBTYWx0ZWRfXyGMQC+TSHqmi PfF8mJx78RvW8YYiPihhV2kW/5THFX/wCF4CYnMBEA+cOHErw3nQTr2vfF/6nZqzUig4gvA6lee mEMUmVzahWG8YDI+W6UdVe+wMd9+mlY/9aJrJqgA4vUV7454oI1cz0japiYecPBeawyc0q3bCDH mUuf0u2mMCRWgNMWxXYCVuJ8LcySa22JE4S9ButssO3nZd3N0rlTuhJo33F6CBmsPsD+KkEaNfh k6FUEOYTlPwbQDlKGtDAbwVxjMyK1ROOPnr/lDX2rHJ382WMY97DZ385Uw/G+Mj4WuPhSOEXEm6 3syEeUrJN27jYxblFCj3fv9GhRZ6yb4NpwQKhQ6M9o9YUXiywk6LL2daxmqK3sCpTBbgKun0l46 5t31QSf9AeAmaXGowbZgKpT4ddt7Qcfnr0JOSi6ya4W0eM2AG3Uuh7hMoqhJOXJ6QxJaq1fkHMX Exaa6uexu9mtDCB/E0w== X-Authority-Analysis: v=2.4 cv=U9uiy+ru c=1 sm=1 tr=0 ts=6a317b0e cx=c_pps a=3Bg1Hr4SwmMryq2xdFQyZA==:117 a=3Bg1Hr4SwmMryq2xdFQyZA==:17 a=FelO9ux0wxsA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=uAbxVGIbfxUO_5tXvNgY:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=dl9XnktawrrMqxjOnboA:9 X-Proofpoint-GUID: 7dT-KaVCJgpJP0gU7MjyAq95-Ynhr3UV X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.125,FMLib:17.12.100.49 definitions=2026-06-16_05,2026-06-15_04,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 impostorscore=0 bulkscore=0 phishscore=0 priorityscore=1501 clxscore=1015 adultscore=0 malwarescore=0 suspectscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606040000 definitions=main-2606160167 On IBM POWER systems, newer processor generations can operate in compatibility modes corresponding to earlier generations. This becomes relevant for nested virtualization, where nested KVM guests may need to run with a specific processor compatibility level. Currently, when running a nested KVM guest (L2) inside a Power11 pSeries logical partition (L1) booted in Power10 compatibility mode, the guest fails to boot while setting 'arch_compat'. This happens because the CPU class is derived from the hardware PVR (via mfspr()), which reflects the physical processor generation (Power11), rather than the effective compatibility mode (Power10). As a result, userspace may request a Power11 arch_compat for the L2 guest. However, the L1 partition, running in Power10 compatibility, has only negotiated support up to Power10 with the Power Hypervisor (L0). When H_GUEST_SET_STATE is invoked with a Power11 Logical PVR, the hypervisor rejects the request, leading to a late guest boot failure: KVM-NESTEDv2: couldn't set guest wide elements [..KVM reg dump..] This situation should be detected earlier and rejected by KVM. Without proper validation, if userspace ignores the error, the guest may continue to boot in Power11 raw mode on a Power10 compatibility host, which should not be allowed. Introduce a validation mechanism that detects unsupported arch_compat values early in the guest initialization path. When an unsupported arch_compat is requested (e.g., Power11 on a Power10 compatibility mode host), kvmppc_set_arch_compat() uses cpu_has_feature(CPU_FTR_P11_PVR) to detect the mismatch and sets arch_compat to PVR_ARCH_INVALID (0xffffffff). This sentinel value is architecturally safe: PAPR specifies that valid logical PVR values must have 0x0f as the first byte, ensuring 0xffffffff lies permanently outside the specification-defined range. Setting this value triggers kvmppc_sanity_check() to mark the vCPU as invalid by setting vcpu->arch.sane to false. On the next vCPU run, kvmppc_vcpu_run_hv() checks this flag and returns -EINVAL, preventing the guest from running with an invalid processor compatibility configuration. With this, when a Power11 arch_compat is requested on a Power10 compatibility mode host, the guest fails early during boot with: error: kvm run failed Invalid argument This provides a much clearer failure mode compared to the previous behavior where the guest could boot in Power11 raw mode (if userspace ignored the error) or fail late during H_GUEST_SET_STATE. Suggested-by: Vaibhav Jain Reviewed-by: Vaibhav Jain Tested-by: Anushree Mathur Acked-by: Gautam Menghani Cc: stable@vger.kernel.org # v6.13+ Signed-off-by: Amit Machhiwal --- Testing: Both Anushree and I have tested the below scenarios: 1. P11 guest on P11 host - Works 2. P10 compat guest on P11 host - Works 3. P11 guest on compat-P10 host - Correctly fails with "Invalid argument" 4. P10 guest on compat-P10 host - Works Changes in v4: * Added documentation for PVR_ARCH_INVALID explaining why 0xffffffff is architecturally safe to use as a sentinel value (PAPR constraint on first byte being 0x0f) [Ritesh] * Updated commit message * v3: https://lore.kernel.org/all/20260609053327.61563-1-amachhiw@linux.ibm.com/ Changes in v3: * Fixed null pointer dereference in kvmppc_sanity_check(): added check for vcpu->arch.vcore before accessing arch_compat, as vcore is NULL for Book3S PR and BookE guests (only Book3S HV uses vcore) [Reported by Sashiko AI] * Added Reviewed-by tag from Vaibhav * v2: https://lore.kernel.org/all/20260608201001.65760-1-amachhiw@linux.ibm.com/ Changes in v2: * Fixed issue where v1 allowed guest to boot in Power11 raw mode when userspace ignored the error, by adding validation in kvmppc_sanity_check() to ensure early failure during vCPU run [Found the issue after posting v1, also reported by Gautam.] * Introduced PVR_ARCH_INVALID constant for marking invalid arch_compat * Dropped all Reviewed-by and Tested-by tags due to code changes; requesting fresh reviews * v1: https://lore.kernel.org/all/20260603141539.47620-1-amachhiw@linux.ibm.com/ Changes in v1: * Moved this patch out of the v3 series [1] as discussed here [2] * Addressed below review comments from Ritesh: - Based the PVR validation on cpu features - Fixed hcall name typo - Stable backport [1] https://lore.kernel.org/all/20260522152744.55251-1-amachhiw@linux.ibm.com/ [2] https://lore.kernel.org/all/20260522152744.55251-2-amachhiw@linux.ibm.com/ --- arch/powerpc/include/asm/reg.h | 12 ++++++++++++ arch/powerpc/kvm/book3s_hv.c | 15 ++++++++++++++- arch/powerpc/kvm/powerpc.c | 4 ++++ 3 files changed, 30 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h index 3449dd2b577d..b9ab9df1e2bc 100644 --- a/arch/powerpc/include/asm/reg.h +++ b/arch/powerpc/include/asm/reg.h @@ -1357,6 +1357,18 @@ #define PVR_ARCH_31 0x0f000006 #define PVR_ARCH_31_P11 0x0f000007 +/* + * Kernel-internal sentinel for invalid processor compatibility modes. + * PAPR specifies that the first byte of a valid logical PVR value is + * 0x0f. So 0xffffffff lies permanently outside the PAPR-defined range + * and is safe to repurpose. KVM stores it in vcpu->arch.arch_compat + * when userspace requests an unsupported compatibility mode (e.g., + * Power11 PVR on a Power11 host booted in Power10 compat). + * kvmppc_sanity_check() detects this and prevents the vCPU from + * running with an unsupported arch_compat. + */ +#define PVR_ARCH_INVALID 0xffffffff + /* Macros for setting and retrieving special purpose registers */ #ifndef __ASSEMBLER__ diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c index 61dbeea317f3..f9380ef65750 100644 --- a/arch/powerpc/kvm/book3s_hv.c +++ b/arch/powerpc/kvm/book3s_hv.c @@ -446,7 +446,19 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat) guest_pcr_bit = PCR_ARCH_300; break; case PVR_ARCH_31: + guest_pcr_bit = PCR_ARCH_31; + break; case PVR_ARCH_31_P11: + /* + * Need to check this for ISA 3.1, as Power10 and + * Power11 share the same PCR. For any subsequent ISA + * versions, this will be taken care of by the guest vs + * host PCR comparison below. + */ + if (!cpu_has_feature(CPU_FTR_P11_PVR)) { + arch_compat = PVR_ARCH_INVALID; + goto out; + } guest_pcr_bit = PCR_ARCH_31; break; default: @@ -469,6 +481,7 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat) return -EINVAL; } +out: spin_lock(&vc->lock); vc->arch_compat = arch_compat; kvmhv_nestedv2_mark_dirty(vcpu, KVMPPC_GSID_LOGICAL_PVR); @@ -479,7 +492,7 @@ static int kvmppc_set_arch_compat(struct kvm_vcpu *vcpu, u32 arch_compat) vc->pcr = (host_pcr_bit - guest_pcr_bit) | PCR_MASK; spin_unlock(&vc->lock); - return 0; + return kvmppc_sanity_check(vcpu); } static void kvmppc_dump_regs(struct kvm_vcpu *vcpu) diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c index 00302399fc37..98de68379b18 100644 --- a/arch/powerpc/kvm/powerpc.c +++ b/arch/powerpc/kvm/powerpc.c @@ -258,6 +258,10 @@ int kvmppc_sanity_check(struct kvm_vcpu *vcpu) if (!vcpu->arch.pvr) goto out; + if (vcpu->arch.vcore && + vcpu->arch.vcore->arch_compat == PVR_ARCH_INVALID) + goto out; + /* PAPR only works with book3s_64 */ if ((vcpu->arch.cpu_type != KVM_CPU_3S_64) && vcpu->arch.papr_enabled) goto out; base-commit: 6b5a2b7d9bc156e505f09e698d85d6a1547c1206 -- 2.50.1 (Apple Git-155)