Re: [PATCH] powerpc/dt_cpu_ftrs: Set CPU_FTR_P11_PVR for Power11 and later processors

[Amit Machhiwal <amachhiw@linux.ibm.com>]

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: stable@vger.kernel.org # v6.13+
> > Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> > ---
> > Related: https://lore.kernel.org/all/20260609053327.61563-1-amachhiw@linux.ibm.com/
> > ---
> > 
> >   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
>