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

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Date:   Mon, 8 Jan 2018 19:49:39 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <nick.desaulniers@...il.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


* Juergen Gross <jgross@...e.com> wrote:

> On 08/01/18 17:10, Peter Zijlstra wrote:
> > 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.
> 
> This only a pointer to the bitmap.

What's the maximum NR_CPUs for configs that can run this code, times 8?

Thanks,

	Ingo