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

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Date:   Fri, 5 Aug 2022 11:47:10 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     ira.weiny@...el.com, Rik van Riel <riel@...riel.com>,
        Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [RFC PATCH 5/5] x86/entry: Store CPU info on exception entry

On 8/5/22 10:30, ira.weiny@...el.com 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.