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Thu, 6 Aug 2026 17:06:57 +0000 (GMT) From: Amit Machhiwal To: linuxppc-dev@lists.ozlabs.org, Madhavan Srinivasan Cc: Vaibhav Jain , Amit Machhiwal , Anushree Mathur , Paolo Bonzini , Nicholas Piggin , Michael Ellerman , "Christophe Leroy (CS GROUP)" , Jonathan Corbet , Shuah Khan , Ritesh Harjani , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Gautam Menghani Subject: [PATCH v7 1/4] KVM: PPC: Introduce KVM_CAP_PPC_COMPAT_CAPS and wire up ioctl Date: Thu, 6 Aug 2026 22:36:42 +0530 Message-ID: <20260806170645.11892-2-amachhiw@linux.ibm.com> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260806170645.11892-1-amachhiw@linux.ibm.com> References: <20260806170645.11892-1-amachhiw@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-Spam-Info: AW1haW4tMjYwODA2MDEzMSBTYWx0ZWRfX1i8d98aaaXYk lOnqE+CT7NWzJWlYS9ulEc3fE+N1uEhjAmz8DwJDX+lWwrdtV+kwo9y1VCRBybc5VvHg/hYEAR0 V/Vy3SC9e8jf9NbjHfqla+rnHkbsCrg= X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODA2MDEzMSBTYWx0ZWRfXw5CDzGHTj1Kk OVIubkRZ9h72hyFgoGyPxXZx/ubTPcrl40ga32AKxmcmzFi1MlBFH7ybuQnIXzzp+p7fkjOzYiV k02/kv6BDJkqryExZLKxRVMzke6hkuz64QJZkGOKrFRbaDBCQhkSbZn3wU1p777YDw9P90jbdlw moZgHmsDLkHWrH0ex8vdXotT1T2mFVwJyzpn6+cgGe0j+SKjnExr1EoyCQKEIePAoInMCpHjkIP FnwHJCAQa8JEHnx8tQjk2V7kdokw5dX43CYnPSE/TWHLNhO25PIlkkdUw9JG3weyE65SyowCCoW 9n4rd+RTIYAIK77sQNq6QRm1oetXZO/kB4POmH7pBLzn2B/xZckjHmuD3t8Msrx0J5omopuGSTs jvxKQQ5c0qRsDXL5CL4pdIn4SNHyJh1fXWkd0jAB6pcipLpdOfv3VPMqcavQh4Bfvj4O812dCZq ouOsVxDl1kIN8iNUrrg== X-Authority-Analysis: v=2.4 cv=SI1ykuvH c=1 sm=1 tr=0 ts=6a74bf3b cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=Sv0fKeRqtYgA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=U7nrCbtTmkRpXpFmAIza:22 a=VnNF1IyMAAAA:8 a=rQyBRTFaFC3FL7QHXY8A:9 X-Proofpoint-ORIG-GUID: xw4h6RMYoy5kFufRhgRZvgjVBJV1WePw X-Proofpoint-GUID: ysCEd5QDaFYDo4jTNYcOiNiQ16EPj3ac X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1176,Hydra:6.1.134,FMLib:17.12.100.49 definitions=2026-08-06_01,2026-08-05_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 bulkscore=0 suspectscore=0 impostorscore=0 spamscore=0 phishscore=0 priorityscore=1501 lowpriorityscore=0 adultscore=0 malwarescore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2606150000 definitions=main-2608060131 Introduce a new capability and ioctl to expose CPU compatibility modes supported by the host processor for nested guests. On IBM POWER systems, newer processor generations (N) can operate in compatibility modes corresponding to earlier generations, like (N-1) and (N-2). This is particularly relevant for nested virtualization, where nested KVM guests may need to run with a specific processor compatibility level. Introduce KVM_CAP_PPC_COMPAT_CAPS capability and the corresponding KVM_PPC_GET_COMPAT_CAPS vm ioctl. The ioctl returns a bitmap describing the compatibility modes supported by the host in respective bit numbers, allowing userspace (e.g., QEMU) to select an appropriate compatibility level when configuring nested KVM guests. The ioctl handling is added in kvm_arch_vm_ioctl() and retrieves host CPU compatibility capabilities via a PowerPC-specific backend implementation when available. The struct kvm_ppc_compat_caps places the 'size' field first so it can be read alone via get_user() before copy_struct_from_user() is called, avoiding pointer arithmetic to locate the size field. The ioctl is defined using _IO so the ioctl number remains stable even if the struct grows in future versions. It uses copy_struct_from_user() and copy_struct_to_user() to provide forward- and backward-compatible extensibility: older userspace passing a smaller struct to a newer kernel gets zero-padded trailing fields, while newer userspace passing a larger struct to an older kernel (usize > ksize) gets sizeof(struct kvm_ppc_compat_caps) written back to host_caps.size so it can retry with the older kernel-supported size, after which the kernel returns -E2BIG. KVM_PPC_COMPAT_CAPS_SIZE_VER0 is defined as a frozen integer constant (24) marking the size of the initial struct version, used as the minimum floor for size field validation, similar to other versioned struct interfaces in the kernel. The 'flags' field is reserved for future use. The kernel rejects any call where flags is non-zero with -EINVAL, preventing garbage values from being baked into ABI permanently. The ioctl returns appropriate error codes: EINVAL for an invalid size or non-zero reserved fields, E2BIG if new userspace provides a larger struct than the kernel knows about (with ksize written back into host_caps.size for the retry), EFAULT for failed copy operations, and ENOTTY if the backend doesn't implement get_compat_caps. Suggested-by: Vaibhav Jain Tested-by: Gautam Menghani Reviewed-by: Gautam Menghani Tested-by: Anushree Mathur Signed-off-by: Amit Machhiwal --- Changes in this version: - KVM_CAP_PPC_COMPAT_CAPS: add hv_enabled guard to align the capability check with ioctl availability; a PR KVM VM on pseries now correctly returns 0 for the capability [Sashiko] arch/powerpc/include/asm/kvm_ppc.h | 1 + arch/powerpc/include/uapi/asm/kvm.h | 8 ++++ arch/powerpc/kvm/powerpc.c | 71 +++++++++++++++++++++++++++++ include/uapi/linux/kvm.h | 3 ++ 4 files changed, 83 insertions(+) diff --git a/arch/powerpc/include/asm/kvm_ppc.h b/arch/powerpc/include/asm/kvm_ppc.h index 0953f2daa466..169ea6a7fbad 100644 --- a/arch/powerpc/include/asm/kvm_ppc.h +++ b/arch/powerpc/include/asm/kvm_ppc.h @@ -319,6 +319,7 @@ struct kvmppc_ops { bool (*hash_v3_possible)(void); int (*create_vm_debugfs)(struct kvm *kvm); int (*create_vcpu_debugfs)(struct kvm_vcpu *vcpu, struct dentry *debugfs_dentry); + int (*get_compat_caps)(struct kvm_ppc_compat_caps *host_caps); }; extern struct kvmppc_ops *kvmppc_hv_ops; diff --git a/arch/powerpc/include/uapi/asm/kvm.h b/arch/powerpc/include/uapi/asm/kvm.h index 077c5437f521..19e53d5ae540 100644 --- a/arch/powerpc/include/uapi/asm/kvm.h +++ b/arch/powerpc/include/uapi/asm/kvm.h @@ -437,6 +437,14 @@ struct kvm_ppc_cpu_char { __u64 behaviour_mask; /* valid bits in behaviour */ }; +/* For KVM_PPC_GET_COMPAT_CAPS */ +struct kvm_ppc_compat_caps { + __u64 size; /* Size of this structure */ + __u64 flags; /* Reserved for future use */ + __u64 compat_capabilities; /* Capabilities supported by the host */ +}; +#define KVM_PPC_COMPAT_CAPS_SIZE_VER0 24 /* sizeof first published struct */ + /* * Values for character and character_mask. * These are identical to the values used by H_GET_CPU_CHARACTERISTICS. diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c index b6b83fe3233f..e64b3cfadd3a 100644 --- a/arch/powerpc/kvm/powerpc.c +++ b/arch/powerpc/kvm/powerpc.c @@ -703,6 +703,13 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) } } break; +#if defined(CONFIG_KVM_BOOK3S_HV_POSSIBLE) + case KVM_CAP_PPC_COMPAT_CAPS: + r = 0; + if (hv_enabled && kvmhv_on_pseries()) + r = 1; + break; +#endif /* CONFIG_KVM_BOOK3S_HV_POSSIBLE */ default: r = 0; break; @@ -2469,6 +2476,70 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg) r = kvm->arch.kvm_ops->svm_off(kvm); break; } + case KVM_PPC_GET_COMPAT_CAPS: { + struct kvm_ppc_compat_caps host_caps = {}; + u64 usize; + + /* + * Read the size field first to drive copy_struct_from_user. + * size must be the first field of the struct. + */ + r = -EFAULT; + if (get_user(usize, (__u64 __user *)argp)) + goto out; + + /* + * Enforce a minimum: reject buffers smaller than the initial + * struct version (VER0). This allows old userspace compiled + * against the original struct to still work on a newer kernel + * that has grown the struct with appended fields. + */ + r = -EINVAL; + if (usize < KVM_PPC_COMPAT_CAPS_SIZE_VER0) + goto out; + + /* + * New userspace with a larger struct called an older kernel. + * Write back ksize in host_caps.size so userspace knows which + * older struct to retry with, then fail with -E2BIG. + */ + if (usize > sizeof(host_caps)) { + host_caps.size = sizeof(host_caps); + r = -EFAULT; + if (put_user(host_caps.size, (__u64 __user *)argp)) + goto out; + r = -E2BIG; + goto out; + } + + /* + * copy_struct_from_user() handles forward/backward compat: + * usize == ksize: verbatim copy + * usize < ksize: zero-pad trailing (old userspace, new kernel) + */ + r = copy_struct_from_user(&host_caps, sizeof(host_caps), + argp, usize); + if (r) + goto out; + + /* Reserved fields must be zero */ + r = -EINVAL; + if (host_caps.flags) + goto out; + + r = -ENOTTY; + if (!kvm->arch.kvm_ops->get_compat_caps) + goto out; + + r = kvm->arch.kvm_ops->get_compat_caps(&host_caps); + if (r) + goto out; + + host_caps.size = sizeof(host_caps); + r = copy_struct_to_user(argp, usize, &host_caps, + sizeof(host_caps), NULL); + break; + } default: { struct kvm *kvm = filp->private_data; r = kvm->arch.kvm_ops->arch_vm_ioctl(filp, ioctl, arg); diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h index 419011097fa8..70e36e6a0ad4 100644 --- a/include/uapi/linux/kvm.h +++ b/include/uapi/linux/kvm.h @@ -997,6 +997,7 @@ struct kvm_enable_cap { #define KVM_CAP_S390_KEYOP 247 #define KVM_CAP_S390_VSIE_ESAMODE 248 #define KVM_CAP_S390_HPAGE_2G 249 +#define KVM_CAP_PPC_COMPAT_CAPS 250 struct kvm_irq_routing_irqchip { __u32 irqchip; @@ -1341,6 +1342,8 @@ struct kvm_s390_keyop { /* Available with KVM_CAP_COUNTER_OFFSET */ #define KVM_ARM_SET_COUNTER_OFFSET _IOW(KVMIO, 0xb5, struct kvm_arm_counter_offset) #define KVM_ARM_GET_REG_WRITABLE_MASKS _IOR(KVMIO, 0xb6, struct reg_mask_range) +/* Available with KVM_CAP_PPC_COMPAT_CAPS */ +#define KVM_PPC_GET_COMPAT_CAPS _IO(KVMIO, 0xb8) /* ioctl for vm fd */ #define KVM_CREATE_DEVICE _IOWR(KVMIO, 0xe0, struct kvm_create_device) -- 2.50.1 (Apple Git-155)