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

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Date:   Sun, 24 Dec 2017 13:02:47 -0500
From:   Nick Desaulniers <nick.desaulniers@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     ghackmann@...gle.com, mka@...gle.com, kees@...gle.com,
        srhines@...gle.com, Nick Desaulniers <nick.desaulniers@...il.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.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: [PATCH] x86: xen: remove the use of VLAIS

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 can define the variable length array (mask) to be a
pointer, and 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).

Signed-off-by: Nick Desaulniers <nick.desaulniers@...il.com>
---
 arch/x86/xen/mmu_pv.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 4d62c07..966976c 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
+		unsigned long *mask;
 	} *args;
 	struct multicall_space mcs;
+	const size_t mc_entry_size = sizeof(args->op) +
+		sizeof(*args->mask) * BITS_TO_LONGS(num_possible_cpus());
 
 	trace_xen_mmu_flush_tlb_others(cpus, info->mm, info->start, info->end);
 
 	if (cpumask_empty(cpus))
 		return;		/* nothing to do */
 
-	mcs = xen_mc_entry(sizeof(*args));
+	mcs = xen_mc_entry(mc_entry_size);
 	args = mcs.args;
 	args->op.arg2.vcpumask = to_cpumask(args->mask);
 
-- 
2.7.4