Re: [PATCH v3] KVM: PPC: Book3S HV: Validate arch_compat against host compatibility mode

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Amit Machhiwal <amachhiw@linux.ibm.com> writes:

>> > diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
>> > index 3449dd2b577d..7472b9522f71 100644
>> > --- a/arch/powerpc/include/asm/reg.h
>> > +++ b/arch/powerpc/include/asm/reg.h
>> > @@ -1356,6 +1356,7 @@
>> >  #define PVR_ARCH_300	0x0f000005
>> >  #define PVR_ARCH_31	0x0f000006
>> >  #define PVR_ARCH_31_P11	0x0f000007
>> > +#define PVR_ARCH_INVALID	0xffffffff
>> 
>> Logical processor version is defined as part of the PAPR spec. We should
>> ensure that this invalid PVR is also documented in the PAPR spec.
>> 
>> If you have already taken care of that, then please confirm and feel free to add:
>
> Regarding the PAPR specification documentation: The PAPR spec documents
> the valid Processor Version Register (PVR) values for each processor
> generation (POWER8, POWER9, POWER10, POWER11, etc.). However, the
> PVR_ARCH_INVALID value (0xffffffff) introduced in this patch series is a
> KVM implementation detail used internally to mark invalid compatibility
> mode requests - it's not an architectural value that would be defined in
> PAPR itself.
>
> The validation logic and the use of PVR_ARCH_INVALID as a sentinel value
> are documented in the kernel code and commit message.
>

But that still worries me on what if PAPR wants to re-use this value for
some other purpose in future. 

BTW, thinking more about it, if we purely want this to be in kernel only,
can we instead add, something like:

     bool kpvr_compat;   /* Does kernel supports this PVR */

rather than re-using & overloading arch_compat which has values that
comes from PAPR spec?

Thoughts?

-ritesh