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Thu, 06 Aug 2026 21:32:50 -0700 (PDT) From: Ritesh Harjani (IBM) To: Amit Machhiwal , 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 , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Gautam Menghani Subject: Re: [PATCH v7 2/4] KVM: PPC: Book3S HV: Implement compat CPU capability retrieval for KVM on PowerVM In-Reply-To: <20260806170645.11892-3-amachhiw@linux.ibm.com> Date: Fri, 07 Aug 2026 10:01:58 +0530 Message-ID: References: <20260806170645.11892-1-amachhiw@linux.ibm.com> <20260806170645.11892-3-amachhiw@linux.ibm.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list Amit Machhiwal writes: > On POWER systems, the host CPU may run in a compatibility mode (e.g., a > Power11 processor operating in Power10 compatibility mode). In such > cases, the effective CPU level exposed to guests differs from the > physical processor generation. > > When running nested KVM guests, QEMU derives the host CPU type using > mfpvr(), which reflects the physical processor version. This can result > in a mismatch between the CPU model selected by QEMU and the > compatibility mode enforced by the host, leading to guest boot failures. > > For example, booting a nested guest on a Power11 LPAR configured in > Power10 compatibility mode fails with: > > KVM-NESTEDv2: couldn't set guest wide elements > [..KVM reg dump..] > > This occurs because QEMU selects a CPU model corresponding to the > physical processor (via mfpvr()), while the host operates in a lower > compatibility mode. As a result, KVM rejects the requested compatibility > level during guest initialization. > > On pseries nestedv2 systems, add support for retrieving host CPU > compatibility capabilities for nested guests on PowerVM. The capability > bitmap reflects the processor modes negotiated between the Power > hypervisor (L0) and the host partition (L1) via the > H_GUEST_GET_CAPABILITIES hcall, but is retrieved from the cached > nested_capabilities value populated during module initialization, > avoiding repeated hypervisor calls. A WARN_ON_ONCE() flags the > unexpected case where nested_capabilities is zero on a nestedv2 system. > The implementation defines KVM-specific capability constants > (KVM_PPC_COMPAT_CAP_POWER9/10/11), masks unsupported bits, and exposes > the result through the KVM_PPC_GET_COMPAT_CAPS ioctl. > > Hook the implementation into the Book3S HV kvmppc_ops so that it can be > invoked by the generic KVM ioctl handling code. > > Suggested-by: Vaibhav Jain > Tested-by: Gautam Menghani > Reviewed-by: Gautam Menghani > Tested-by: Anushree Mathur > Signed-off-by: Amit Machhiwal > --- > arch/powerpc/include/uapi/asm/kvm.h | 10 ++++++++++ > arch/powerpc/kvm/book3s_hv.c | 20 ++++++++++++++++++++ > 2 files changed, 30 insertions(+) Looks good to me. Please feel free to add: Reviewed-by: Ritesh Harjani (IBM)