On 2026/08/07 10:01 AM, Ritesh Harjani wrote: > Amit Machhiwal <[email protected]> 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 <[email protected]> > > Tested-by: Gautam Menghani <[email protected]> > > Reviewed-by: Gautam Menghani <[email protected]> > > Tested-by: Anushree Mathur <[email protected]> > > Signed-off-by: Amit Machhiwal <[email protected]> > > --- > > 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) <[email protected]>
Thanks for the review, Ritesh! ~Amit
