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Thu, 6 Aug 2026 17:07:15 +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 4/4] KVM: PPC: Document KVM_PPC_GET_COMPAT_CAPS ioctl Date: Thu, 6 Aug 2026 22:36:45 +0530 Message-ID: <20260806170645.11892-5-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: AW1haW4tMjYwODA2MDEzMSBTYWx0ZWRfX/27nE8Rhlb3C UYsmk+BQeItAWU7usouu63M6HKB4a9Nr07hPN/wJlslNuGEjFM33WZIJmx2rlu3s7QBDaoIYPWH wgzl30LN6LnsBNd9Z6WDscjlK8Y8c4w= X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwODA2MDEzMSBTYWx0ZWRfXxvjbhlhYrdMZ Yc/ITkwUrMM5WzihO8hRozRs3Kv3sylx8HZPnbIWKpTV5+R4yAkNUzophI5a5zA5A+nAKwCOzB6 PR+N/MbH5s0WQaoOjd1ilxk5dM3PEE4eg83x8eS83ugCIbJ5Os8C8W90X3a+gYLdUDa2ay2tOye zwb3sPp2qFd/pTP6PMhxEwyy1JngDyCoX3g2+3ZdYE2DUdOG9fKPmQyPztF8g1hFDHMRc2BDAke rWaY0ietaQkXKw67I6AEgxFnE2ehXGd/e3nZN0v9QJQn8zpWbFedQR0T8LMjLlesBqMvXfZDYQJ 1LALgY+Tq0ohaTuOyu3Or1A2u6bEQtFOSSj3K23tF4itDbyUebLtfh8ugNMkD7vde1V2XoMtRwL sppm58GnWreTzoixx6VIkFIRLaJ4Bm4ZlpadceqWPHJZ4c9LuV98az5ts9gjFl+eSdE6Zuiu5XX jpSaVW262rUGYkQ2wlg== X-Authority-Analysis: v=2.4 cv=SI1ykuvH c=1 sm=1 tr=0 ts=6a74bf4c 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=hQeoMbdVOOR6M4iuoH8A:9 X-Proofpoint-ORIG-GUID: 6616m_niNNu7FcKrIYBn4hbKJwqXpNWC X-Proofpoint-GUID: uNKFwZfwU-lddH9JFZG3s1Fy6uF6DjKt 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 Add documentation for the KVM_PPC_GET_COMPAT_CAPS ioctl to the KVM API documentation. The ioctl exposes host processor compatibility modes supported for nested KVM guests on PowerPC systems. The documentation covers error code descriptions including E2BIG for forward compatibility, the extensible size-based versioning contract using KVM_PPC_COMPAT_CAPS_SIZE_VER0, the rationale for rejecting non-zero reserved fields to prevent ABI ambiguity, bit numbering clarification for IBM MSB-0 convention, and KVM-specific capability bit constants. Tested-by: Gautam Menghani Reviewed-by: Gautam Menghani Tested-by: Anushree Mathur Signed-off-by: Amit Machhiwal --- Documentation/virt/kvm/api.rst | 79 ++++++++++++++++++++++++++++++++++ 1 file changed, 79 insertions(+) diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst index e3003a241d5b..22fedb0aa34b 100644 --- a/Documentation/virt/kvm/api.rst +++ b/Documentation/virt/kvm/api.rst @@ -6566,6 +6566,85 @@ KVM_S390_KEYOP_SSKE Sets the storage key for the guest address ``guest_addr`` to the key specified in ``key``, returning the previous value in ``key``. +4.145 KVM_PPC_GET_COMPAT_CAPS +----------------------------- +:Capability: KVM_CAP_PPC_COMPAT_CAPS +:Architectures: powerpc +:Type: vm ioctl +:Parameters: struct kvm_ppc_compat_caps (in/out) +:Returns: 0 on success, negative value on failure + +Errors include: + + ======== ============================================================ + EFAULT if ``struct kvm_ppc_compat_caps`` cannot be read from or + written to userspace + EINVAL if the ``size`` field is smaller than + ``KVM_PPC_COMPAT_CAPS_SIZE_VER0``, if the ``flags`` field + is non-zero, or if the backend fails to retrieve or map + CPU compatibility capabilities + E2BIG if ``size`` is larger than the kernel's struct size + (new userspace on old kernel); the kernel writes back its + own struct size into the ``size`` field so userspace can + retry with the correct size + ENOTTY if the backend does not implement the ``get_compat_caps`` + operation (e.g., on non-HV KVM implementations where the + required KVM operations are not available) + ======== ============================================================ + +IBM POWER system server-based processors provide a compatibility mode feature +where an Nth generation processor can operate in modes consistent with earlier +generations such as (N-1) and (N-2). + +This ioctl provides userspace with information about the CPU compatibility modes +supported by the current host processor for booting the nested KVM guests on +KVM on PowerNV (nested API v1) and KVM on PowerVM (nested API v2) platforms. + +:: + + struct kvm_ppc_compat_caps { + __u64 size; /* Size of this structure */ + __u64 flags; /* Reserved for future use, must be 0 */ + __u64 compat_capabilities; /* Capabilities supported by the host */ + }; + +Before calling this ioctl, userspace must set the ``size`` field to +``sizeof(struct kvm_ppc_compat_caps)`` and zero the ``flags`` field. +The kernel rejects non-zero ``flags`` with ``-EINVAL`` to prevent +uninitialized stack values from being silently accepted, keeping the +field available for future use without ABI ambiguity. + +The ioctl uses ``copy_struct_from_user()`` and ``copy_struct_to_user()`` +to support extensible versioning: if userspace passes a struct smaller +than the current kernel version (``size >= KVM_PPC_COMPAT_CAPS_SIZE_VER0``), +the kernel zero-pads unknown trailing fields. If userspace passes a larger +struct (``size > sizeof(struct kvm_ppc_compat_caps)``), the kernel writes +back its own struct size into the ``size`` field and returns ``-E2BIG``, +allowing userspace to discover the kernel's struct size and retry. +``KVM_PPC_COMPAT_CAPS_SIZE_VER0`` (24) is a frozen constant marking the +size of the initial struct version. + +The ``compat_capabilities`` bit field describes the processor compatibility +modes supported by the host. The following bits indicate support for specific +processor modes (using IBM's MSB-0 convention where bit 0 is the most +significant bit): + +- ``KVM_PPC_COMPAT_CAP_POWER9`` (bit 1) -- KVM guests can run in Power9 processor mode +- ``KVM_PPC_COMPAT_CAP_POWER10`` (bit 2) -- KVM guests can run in Power10 processor mode +- ``KVM_PPC_COMPAT_CAP_POWER11`` (bit 3) -- KVM guests can run in Power11 processor mode + +.. note:: + + The bit numbering above uses IBM's MSB-0 convention (bit 0 is the most + significant bit). In the actual implementation, these are defined as: + + - ``KVM_PPC_COMPAT_CAP_POWER9`` = ``(1ULL << 62)`` + - ``KVM_PPC_COMPAT_CAP_POWER10`` = ``(1ULL << 61)`` + - ``KVM_PPC_COMPAT_CAP_POWER11`` = ``(1ULL << 60)`` + + Userspace should use the defined constants from ```` rather + than hardcoding bit positions. + .. _kvm_run: 5. The kvm_run structure -- 2.50.1 (Apple Git-155)