Re: [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry

[Dave Hansen]

On 8/5/22 10:30, ira.weiny@xxxxxxxxx wrote:
> +static inline void arch_save_aux_pt_regs(struct pt_regs *regs)
> +{
> +	struct pt_regs_auxiliary *aux_pt_regs = &to_extended_pt_regs(regs)->aux;
> +
> +	aux_pt_regs->cpu = raw_smp_processor_id();
> +}

This is in a fast path that all interrupt and exception entry uses.  So,
I was curious what the overhead is.

Code generation in irqentry_enter() gets a _bit_ more complicated
because arch_save_aux_pt_regs() has to be done on the way out of the
function and the compiler can't (for instance) do a

	mov    $0x1,%eax
	ret

to return.  But, the gist of the change is still only two instructions
that read a pretty hot, read-only per-cpu cacheline:

	mov    %gs:0x7e21fa4a(%rip),%eax        # 15a38 <cpu_number>
	mov    %eax,-0x8(%rbx)

That doesn't seem too bad.