Hi Christophe,
Thanks for reviewing this patch. Please find my response inline.
On 2026/06/16 07:39 AM, Christophe Leroy (CS GROUP) wrote:
>
>
> Le 14/06/2026 à 19:34, Amit Machhiwal a écrit :
> > When using device tree CPU features (dt-cpu-ftrs), the kernel bypasses
> > the traditional cputable-based CPU identification and instead derives
> > CPU features from the device tree's "ibm,powerpc-cpu-features" node
> > provided by firmware.
> >
> > However, CPU_FTR_P11_PVR is a kernel-internal feature flag used to
> > identify Power11 and later processors, and is not represented in the
> > device tree's ISA feature set. While ISA v3.1 support (indicated by
> > CPU_FTR_ARCH_31) is present on both Power10 and Power11, the
> > CPU_FTR_P11_PVR flag is specifically needed by code that must
> > distinguish between Power10 and Power11 processors.
> >
> > Without this flag set, code that checks for Power11 using
> > cpu_has_feature(CPU_FTR_P11_PVR) will incorrectly return false on
> > Power11+ systems using dt-cpu-ftrs, leading to incorrect behavior.
> >
> > This issue manifests specifically in powernv environments (bare-metal
> > or QEMU TCG with powernv machine type), where skiboot/OPAL firmware
> > provides the "ibm,powerpc-cpu-features" node, causing the kernel to
> > use dt-cpu-ftrs. The issue does not affect pseries guests, where SLOF
> > firmware does not provide this node, causing the kernel to fall back
> > to the traditional cputable path (identify_cpu) which correctly sets
> > CPU_FTR_P11_PVR during PVR-based CPU identification.
> >
> > In powernv TCG guests, the missing flag causes KVM code to trigger
> > warnings when attempting to create KVM guests, as cpu_features shows
> > 0x000c00eb8f4fb187 (missing bit 53) instead of the correct
> > 0x002c00eb8f4fb187 (with bit 53 set).
> >
> > Fix this by setting CPU_FTR_P11_PVR for all processors with
> > PVR >= PVR_POWER11 when ISA v3.1 support is detected in
> > cpufeatures_setup_start(). This approach ensures forward
> > compatibility with future processor generations.
> >
> > Fixes: 96e266e3bcd6 ("KVM: PPC: Book3S HV: Add Power11 capability support
> > for Nested PAPR guests")
> > Cc: [email protected] # v6.13+
> > Signed-off-by: Amit Machhiwal <[email protected]>
> > ---
> > Related:
> > https://lore.kernel.org/all/[email protected]/
> > ---
> >
> > arch/powerpc/kernel/dt_cpu_ftrs.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> >
> > diff --git a/arch/powerpc/kernel/dt_cpu_ftrs.c
> > b/arch/powerpc/kernel/dt_cpu_ftrs.c
> > index 3af6c06af02f..e5853daa6a48 100644
> > --- a/arch/powerpc/kernel/dt_cpu_ftrs.c
> > +++ b/arch/powerpc/kernel/dt_cpu_ftrs.c
> > @@ -704,6 +704,15 @@ static void __init cpufeatures_setup_start(u32 isa)
> > if (isa >= ISA_V3_1) {
> > cur_cpu_spec->cpu_features |= CPU_FTR_ARCH_31;
> > cur_cpu_spec->cpu_user_features2 |= PPC_FEATURE2_ARCH_3_1;
> > +
> > + /*
> > + * CPU_FTR_P11_PVR is a kernel-internal flag to identify
> > + * Power11 and later processors. While ISA v3.1 is supported
> > + * by Power10+, this flag specifically indicates Power11+
> > + * for code that needs to distinguish between P10 and P11.
> > + */
> > + if (PVR_VER(mfspr(SPRN_PVR)) >= PVR_POWER11)
>
> Are we sure this test will always be correct ?
>
> For instance PVR_PA6T is higher than PVR_POWER11 allthough it is not ISA 3.1
>
> Wouldn't is be cleaner and safer to just do:
>
> PVR_VER(mfspr(SPRN_PVR)) == PVR_POWER11
You're absolutely right to point out the PVR ordering concern. But PA6T
cannot actually reach this path because we're already gated by:
if (isa >= ISA_V3_1)
and PA6T does not implement ISA v3.1.
My rationale for using `>= PVR_POWER11` is that `CPU_FTR_P11_PVR` is
intended to be included for Power11 and later processors, not just
Power11 itself, as it identifies a CPU feature. Using `== PVR_POWER11`
would mean we'd need to revisit this code for every future generation.
This approach is consistent with existing kernel code. For example, in
arch/powerpc/perf/hv-gpci.c:
/* sysinfo interface files are only available for power10 and above platforms
*/
if (PVR_VER(mfspr(SPRN_PVR)) >= PVR_POWER10)
add_sysinfo_interface_files();
Also, I couldn't find any current users of `PVR_PA6T` or `PVR_BE` in the
kernel tree, so there doesn't appear to be a present-day ISA v3.1+
example where the comparison would misidentify a processor.
Please let me know your further thoughts on this.
Thanks,
Amit
>
> > + cur_cpu_spec->cpu_features |= CPU_FTR_P11_PVR;
> > }
> > }
> >
> > base-commit: 424280953322cf66314f3ba5e2d1ef345f21c770
>
