linux-kernel - Re: [RFC PATCH v2 1/2] x86/speculation: Allow per-process control of when to issue IBPB

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Date:   Mon, 03 May 2021 10:48:21 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Anand K Mistry <amistry@...gle.com>, x86@...nel.org
Cc:     joelaf@...gle.com, asteinhauser@...gle.com, bp@...en8.de,
        Anand K Mistry <amistry@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ben Segall <bsegall@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jay Lang <jaytlang@....edu>, Jens Axboe <axboe@...nel.dk>,
        Juri Lelli <juri.lelli@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Mel Gorman <mgorman@...e.de>, Mike Rapoport <rppt@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tony Luck <tony.luck@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 1/2] x86/speculation: Allow per-process control of when to issue IBPB

Anand,

On Thu, Apr 29 2021 at 18:44, Anand K. Mistry wrote:
>  
> -static inline unsigned long mm_mangle_tif_spec_ib(struct task_struct *next)
> +static inline unsigned long mm_mangle_tif_spec_ib_on_leave(
> +	struct task_struct *next)
>  {
>  	unsigned long next_tif = task_thread_info(next)->flags;
> -	unsigned long ibpb = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_IBPB;
> +	unsigned long spec_disabled =
> +		(next_tif >> TIF_SPEC_IB) & ~(next_tif >> TIF_NO_IBPB_LEAVE);
> +	unsigned long ibpb = spec_disabled & LAST_USER_MM_IBPB;
>  
>  	return (unsigned long)next->mm | ibpb;
>  }
>  
> +static inline bool ibpb_on_entry(struct task_struct *next)
> +{
> +	unsigned long next_tif = task_thread_info(next)->flags;
> +	unsigned long spec_disabled =
> +		(next_tif >> TIF_SPEC_IB) & ~(next_tif >> TIF_NO_IBPB_ENTRY);
> +	return spec_disabled & 1;
> +}

Why exactly do we need _three_ TIF bits and this non-intuitive inverted
logic?

The existing mode is: Issue IBPB when scheduling in and when scheduling out.

Ergo the obvious approach for making it more finegrained is to split the
existing TIF_SPEC_IB into TIF_SPEC_IB_IN and TIF_SPEC_IB_OUT and have
the existing mode both bits set.

Thanks,

        tglx