linux-kernel - Re: [PATCH v2] x86: xen: remove the use of VLAIS

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Date:   Mon, 8 Jan 2018 17:10:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <nick.desaulniers@...il.com>
Cc:     Juergen Gross <jgross@...e.com>, ghackmann@...gle.com,
        mka@...gle.com, kees@...gle.com, srhines@...gle.com,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: xen: remove the use of VLAIS

On Sat, Jan 06, 2018 at 01:39:48PM -0800, Nick Desaulniers wrote:
> Variable Length Arrays In Structs (VLAIS) is not supported by Clang, and
> frowned upon by others.
> 
> https://lkml.org/lkml/2013/9/23/500
> 
> Here, the VLAIS was used because the size of the bitmap returned from
> xen_mc_entry() depended on possibly (based on kernel configuration)
> runtime sized data. Rather than declaring args as a VLAIS then calling
> sizeof on *args, we calculate the appropriate sizeof args manually.
> Further, we can get rid of the #ifdef's and rely on num_possible_cpus()
> (thanks to a helpful checkpatch warning from an earlier version of this
> patch).
> 
> Suggested-by: Juergen Gross <jgross@...e.com>
> Signed-off-by: Nick Desaulniers <nick.desaulniers@...il.com>
> ---
> Changes in v2:
> * Change mask to us DECLARE_BITMAP instead of pointer, as suggested.
> * Update commit message to remove mention of pointer.
> * Update sizeof calculation to work with array rather than pointer.
> 
>  arch/x86/xen/mmu_pv.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 4d62c07..d850762 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -1325,20 +1325,18 @@ static void xen_flush_tlb_others(const struct cpumask *cpus,
>  {
>  	struct {
>  		struct mmuext_op op;
> -#ifdef CONFIG_SMP
> -		DECLARE_BITMAP(mask, num_processors);
> -#else
>  		DECLARE_BITMAP(mask, NR_CPUS);
> -#endif
>  	} *args;

Why is it OK for Xen to place this bitmap on-stack in the first place?
That NR_CPUS thing can be fairly huge.