Re: [PATCH v4 02/10] x86/cpu/topology: Add CPU type to struct cpuinfo_topology

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On Fri, Oct 18, 2024 at 11:28:31AM -0500, Mario Limonciello wrote:
> On 9/30/2024 09:47, Pawan Gupta wrote:
> > Sometimes it is required to take actions based on if a CPU is a performance
> > or efficiency core. As an example, intel_pstate driver uses the Intel
> > core-type to determine CPU scaling. Also, some CPU vulnerabilities only
> > affect a specific CPU type, like RFDS only affects Intel Atom. Hybrid
> > systems that have variants P+E, P-only(Core) and E-only(Atom), it is not
> > straightforward to identify which variant is affected by a type specific
> > vulnerability.
> > 
> > Such processors do have CPUID field that can uniquely identify them. Like,
> > P+E, P-only and E-only enumerates CPUID.1A.CORE_TYPE identification, while
> > P+E additionally enumerates CPUID.7.HYBRID. Based on this information, it
> > is possible for boot CPU to identify if a system has mixed CPU types.
> > 
> > Add a new field hw_cpu_type to struct cpuinfo_topology that stores the
> > hardware specific CPU type. This saves the overhead of IPIs to get the CPU
> > type of a different CPU. CPU type is populated early in the boot process,
> > before vulnerabilities are enumerated.
> > 
> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > ---
> >   arch/x86/include/asm/cpu.h            |  6 ++++++
> >   arch/x86/include/asm/processor.h      | 11 +++++++++++
> >   arch/x86/include/asm/topology.h       |  8 ++++++++
> >   arch/x86/kernel/cpu/debugfs.c         |  1 +
> >   arch/x86/kernel/cpu/intel.c           |  5 +++++
> >   arch/x86/kernel/cpu/topology_common.c | 11 +++++++++++
> >   6 files changed, 42 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/cpu.h b/arch/x86/include/asm/cpu.h
> > index aa30fd8cad7f..2244dd86066a 100644
> > --- a/arch/x86/include/asm/cpu.h
> > +++ b/arch/x86/include/asm/cpu.h
> > @@ -32,6 +32,7 @@ extern bool handle_user_split_lock(struct pt_regs *regs, long error_code);
> >   extern bool handle_guest_split_lock(unsigned long ip);
> >   extern void handle_bus_lock(struct pt_regs *regs);
> >   u8 get_this_hybrid_cpu_type(void);
> > +u32 intel_native_model_id(struct cpuinfo_x86 *c);
> >   #else
> >   static inline void __init sld_setup(struct cpuinfo_x86 *c) {}
> >   static inline bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> > @@ -50,6 +51,11 @@ static inline u8 get_this_hybrid_cpu_type(void)
> >   {
> >   	return 0;
> >   }
> > +
> > +static u32 intel_native_model_id(struct cpuinfo_x86 *c)
> > +{
> > +	return 0;
> > +}
> >   #endif
> >   #ifdef CONFIG_IA32_FEAT_CTL
> >   void init_ia32_feat_ctl(struct cpuinfo_x86 *c);
> > diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
> > index 4a686f0e5dbf..61c8336bc99b 100644
> > --- a/arch/x86/include/asm/processor.h
> > +++ b/arch/x86/include/asm/processor.h
> > @@ -105,6 +105,17 @@ struct cpuinfo_topology {
> >   	// Cache level topology IDs
> >   	u32			llc_id;
> >   	u32			l2c_id;
> > +
> > +	// Hardware defined CPU-type
> > +	union {
> > +		u32		hw_cpu_type;
> > +		struct {
> > +			/* CPUID.1A.EAX[23-0] */
> > +			u32	intel_core_native_model_id:24;
> > +			/* CPUID.1A.EAX[31-24] */
> > +			u32	intel_core_type:8;
> > +		};
> > +	};
> >   };
> >   struct cpuinfo_x86 {
> > diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
> > index aef70336d624..faf7cb7f7d7e 100644
> > --- a/arch/x86/include/asm/topology.h
> > +++ b/arch/x86/include/asm/topology.h
> > @@ -114,6 +114,12 @@ enum x86_topology_domains {
> >   	TOPO_MAX_DOMAIN,
> >   };
> > +enum x86_topology_hw_cpu_type {
> > +	TOPO_HW_CPU_TYPE_UNKNOWN	= 0,
> > +	TOPO_HW_CPU_TYPE_INTEL_ATOM	= 0x20,
> > +	TOPO_HW_CPU_TYPE_INTEL_CORE	= 0x40,
> > +};
> 
> This isn't exactly generic.  Unless you have a strong need to know "Atom"

The goal was not to have generic cpu_type here, but the actual CPU type
that hardware enumerates. I was asked to prepend "hw_" to cpu_type to make
is clear that this is hardware defined, and to leave scope for generic
cpu_type, if we add those in future.

> instead of "Efficient" or "Core" instead of "Performance" I think it would
> be better to do this as:
>
> enum x86_topology_hw_core_type {
> 	TOPO_HW_CORE_TYPE_UNKNOWN	= 0,
> 	TOPO_HW_CORE_TYPE_PERFORMANT,
> 	TOPO_HW_CORE_TYPE_EFFICIENT,
> };
>
> Then you can do the mapping of 0x20 = Efficient and 0x40 = performant in the
> Intel topology lookup function.

I can add a lookup function, but I wanted to understand the use case of
generic cpu_type. If we always have to lookup and map the cpu_type, then
why not have the actual cpu_type in the first place?

One case where generic cpu_type can be useful is when we expose them to
userspace, which I think is inevitable. Overall I am fine with adding generic
cpu type. It may also make sense to have separate accessors for generic and
and hardware defined cpu_type, and the generic ones when we actually have a
use case. Thoughts?

> After you land the series we can do something similar to move AMD code
> around and map it out to the right generic mapping.