linux-kernel - Re: [PATCH v1 04/12] x86/mce: Remove noinstr annotation from mce

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Message-ID: <YbHzuJmDXJ/ryWR6@hirez.programming.kicks-ass.net>
Date:   Thu, 9 Dec 2021 13:16:56 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tony Luck <tony.luck@...el.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 04/12] x86/mce: Remove noinstr annotation from
 mce_setup()

On Wed, Dec 08, 2021 at 12:13:35PM +0100, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> Instead, sandwitch around the call which is done in noinstr context and
> mark the caller - mce_gather_info() - as noinstr.
> 
> Signed-off-by: Borislav Petkov <bp@...e.de>
> ---
>  arch/x86/kernel/cpu/mce/core.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index 87a277fc80b4..f61f14faa532 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -127,7 +127,7 @@ static struct irq_work mce_irq_work;
>  BLOCKING_NOTIFIER_HEAD(x86_mce_decoder_chain);
>  
>  /* Do initial initialization of a struct mce */
> -noinstr void mce_setup(struct mce *m)
> +void mce_setup(struct mce *m)
>  {
>  	memset(m, 0, sizeof(struct mce));
>  	m->cpu = m->extcpu = smp_processor_id();
> @@ -430,9 +430,15 @@ static noinstr void mce_wrmsrl(u32 msr, u64 v)
>   * check into our "mce" struct so that we can use it later to assess
>   * the severity of the problem as we read per-bank specific details.
>   */
> -static inline void mce_gather_info(struct mce *m, struct pt_regs *regs)
> +static noinstr void mce_gather_info(struct mce *m, struct pt_regs *regs)
>  {
> +	/*
> +	 * Enable instrumentation around mce_setup() which calls external
> +	 * facilities.

Yeah, that's what it does; but *why* is that correct? I'm thinking we're
well past the exception entry code and are only using noinstr as a means
to limit the amount of code in the MCE handler?

> +	 */
> +	instrumentation_begin();
>  	mce_setup(m);
> +	instrumentation_end();
>  
>  	m->mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
>  	if (regs) {
> -- 
> 2.29.2
>