On 02/05/26 7:30 PM, Amit Machhiwal wrote:
> On POWER systems, newer processor generations can operate in compatibility
> modes corresponding to earlier generations (e.g., a Power11 system running
> in Power10 compatibility mode). In such cases, the effective CPU level
> exposed to guests differs from the physical processor generation.
>
> This creates issues for nested virtualization. When booting a nested KVM
> guest, QEMU may derive the CPU model from the raw hardware PVR and attempt
> to configure the guest accordingly. However, the host is constrained by the
> compatibility level negotiated with the hypervisor, and requests exceeding
> that level are rejected by KVM, leading to guest boot failures such as:
>
> KVM-NESTEDv2: couldn't set guest wide elements
>
> This series addresses the issue in two ways:
>
> 1. Do not silently fall back to raw mode when KVM rejects a requested
> compatibility level during CAS. Instead, propagate the error so invalid
> configurations are visible and fail early.
>
> 2. Query the effective CPU compatibility modes supported by the host via
> KVM and use this information to select an appropriate CPU model for
> nested guests.
>
> With these changes, QEMU avoids masking KVM errors and ensures that nested
> guests are configured with CPU models consistent with the host
> compatibility mode, allowing them to boot correctly.
>
> Patch summary:
> [1/3] hw/ppc/spapr: Do not fallback to raw mode when KVM rejects compat
> [2/3] [DO_NOT_MERGE] linux-headers: Add uapi header changes
> [3/3] target/ppc/kvm: Use host compatibility mode for nested guests
>
> Changes in v2:
> - Patch 3: Guard compatibility mode code with #if defined(TARGET_PPC64)
> to fix compilation for ppc32 targets. The POWER9/10/11 PVR constants
> are only defined for 64-bit builds, and compatibility modes are only
> relevant for 64-bit systems.
>
> Tested on:
> - Power11 pSeries LPAR in Power10 compatibility mode
> - Power10 PowerNV and QEMU PowerNV 11 TCG L0 host
>
> CI test results: https://gitlab.com/amachhiw/qemu/-/pipelines/2494987253
>
> Note: Patch 2 is marked DO_NOT_MERGE as it contains linux-headers updates
> that will be synced separately once the corresponding kernel patches are
> merged.
>
> The corresponding Linux patches have been posted [1]
>
> [1] https://lore.kernel.org/all/20260430054906.94431-1-amachhiw@linux.ibm.com/
>
> Amit Machhiwal (3):
> hw/ppc/spapr: Do not fallback to raw mode when KVM rejects compat
> [DO_NOT_MERGE] linux-headers: Add uapi header changes
> target/ppc/kvm: Use host compatibility mode for nested guests
>
> hw/ppc/spapr_hcall.c | 9 ++++++
> linux-headers/asm-powerpc/kvm.h | 7 ++++
> linux-headers/linux/kvm.h | 3 ++
> target/ppc/kvm.c | 57 +++++++++++++++++++++++++++++++++
> 4 files changed, 76 insertions(+)
>
>
> base-commit: 3d626609ccae61a2e552bccd59c7a0931bab8261
Hi Amit,
I tried booting up a guest on P11 lpar booted with P10 compat mode
applying your patch along with the linux patch series and it has been
working perfectly fine.
Host lscpu:
lscpu
Architecture: ppc64le
Byte Order: Little Endian
CPU(s): 80
On-line CPU(s) list: 0-79
Model name: POWER10 (architected), altivec supported
Guest lscpu:
lscpu
Architecture: ppc64le
Byte Order: Little Endian
CPU(s): 10
On-line CPU(s) list: 0-9
Model name: POWER10 (architected), altivec supported
Feel free to add :
Tested-by: Anushree Mathur <anushree.mathur@linux.ibm.com>
Thank you!
Anushree Mathur