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

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Date:   Mon, 8 Jan 2018 11:20:58 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <nick.desaulniers@...il.com>
Cc:     Juergen Gross <jgross@...e.com>, 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 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.

-boris