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

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Message-ID: <c311d554-4f77-f7ef-4f37-c2aa3fde783f@oracle.com>
Date:   Mon, 8 Jan 2018 11:35:41 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nick Desaulniers <nick.desaulniers@...il.com>
Cc:     x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org, ghackmann@...gle.com, mka@...gle.com,
        Ingo Molnar <mingo@...hat.com>, srhines@...gle.com,
        kees@...gle.com, xen-devel@...ts.xenproject.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH v2] x86: xen: remove the use of VLAIS

On 01/08/2018 11:28 AM, Juergen Gross wrote:
> 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.
>


It's the full array, isn't it?

#define DECLARE_BITMAP(name,bits) \
        unsigned long name[BITS_TO_LONGS(bits)]

<pause>

OK, it *is* a pointer. Sigh...

-boris