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

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Date:   Tue, 9 Jan 2018 06:35:54 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Ingo Molnar <mingo@...nel.org>
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

On 08/01/18 19:49, Ingo Molnar wrote:
> 
> * 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?

Why does this matter? args is a pointer only, so it occupies 8 bytes of
the stack. The structure is only for type correctness.


Juergen