Re: [PATCH RESEND v4 2/3] x86/apic: Introduce a variable to track the number of EILVT registers

[Naveen N Rao <naveen@kernel.org>]

On Mon, Sep 21, 2026 at 10:36:10PM -0700, Borislav Petkov wrote:
> On Thu, Sep 03, 2026 at 02:58:40PM +0530, Naveen N Rao (AMD) wrote:
> > diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> > index 90025451ace2..c990f403ec7f 100644
> > --- a/arch/x86/kernel/apic/apic.c
> > +++ b/arch/x86/kernel/apic/apic.c
> > @@ -410,6 +410,15 @@ int setup_APIC_eilvt(u8 offset, u8 vector, u8 msg_type, u8 mask)
> >  }
> >  EXPORT_SYMBOL_GPL(setup_APIC_eilvt);
> >  
> > +static __init void init_eilvt(void)
> > +{
> > +	if (cpu_feature_enabled(X86_FEATURE_EXTAPIC))
> > +		apic->eilvt_regs_count = APIC_EFEAT_XLC(apic_read(APIC_EFEAT));
> > +
> > +	if (!apic->eilvt_regs_count && boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
> > +		apic->eilvt_regs_count = APIC_EILVT_NR_AMD_10H;
> 
> Why not simply:
> 
> 	if (cpu_feature_enabled(X86_FEATURE_EXTAPIC))
> 		apic->eilvt_regs_count = APIC_EFEAT_XLC(apic_read(APIC_EFEAT));
> 	else if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD)
> 		apic->eilvt_regs_count = APIC_EILVT_NR_AMD_10H;

Cause I didn't want to risk changing the current default EILVT register 
count for some ~2 decade old processor that may be advertising 
X86_FEATURE_EXTAPIC but encoding a count of zero in APIC_EFEAT.

I did look up BKDGs for some of the older processors, but couldn't work 
out if this ever was a problem. This was discussed here:
https://lore.kernel.org/all/agckJQQuLsARA4ul@blrnaveerao1/

> And now I see the big picture - you have a single logical patch - add
> eilvt_regs_count - and the rest of the code is reorg and cleanup.

Isn't that the case for most patch series? ;)

> 
> So why are there three patches?

Right, the split reflects how I viewed the changes, and to hopefully 
simplify the review. Looks like I got the split wrong as it looks to 
have been counter-productive here.


Thanks,
- Naveen