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

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Message-ID: <62287687-99c2-b306-d1fc-7dc87fd38eb6@suse.com>
Date:   Mon, 8 Jan 2018 17:28:05 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <nick.desaulniers@...il.com>
Cc:     ghackmann@...gle.com, mka@...gle.com, kees@...gle.com,
        srhines@...gle.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 17:20, Boris Ostrovsky wrote:
> On 01/08/2018 11:10 AM, 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.
> 
> Err... right. Now it's even worse than it was before, when the array was
> potentially smaller. I'll drop this.

No, its only the pointer to the struct, not the struct itself.


Juergen