linux-kernel - [PATCH] tracing/x86/xen: Remove zero data size trace events trace_xen_mmu_flush_tlb{

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Date:   Wed, 9 May 2018 14:46:05 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jingjie Jiang <jingjie.jiang@...cle.com>,
        Mukesh Rathor <mukesh.rathor@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        stable@...r.kernel.org
Subject: [PATCH] tracing/x86/xen: Remove zero data size trace events
 trace_xen_mmu_flush_tlb{_all}


From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>

Doing an audit of trace events, I discovered two trace events in the xen
subsystem that use a hack to create zero data size trace events. This is not
what trace events are for. Trace events add memory footprint overhead, and
if all you need to do is see if a function is hit or not, simply make that
function noinline and use function tracer filtering.

Worse yet, the hack used was:

 __array(char, x, 0)

Which creates a static string of zero in length. There's assumptions about
such constructs in ftrace that this is a dynamic string that is nul
terminated. This is not the case with these tracepoints and can cause
problems in various parts of ftrace.

Nuke the trace events!

Cc: stable@...r.kernel.org
Fixes: 95a7d76897c1e ("xen/mmu: Use Xen specific TLB flush instead of the generic one.")
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
---
 arch/x86/xen/mmu.c         |  4 +---
 arch/x86/xen/mmu_pv.c      |  4 +---
 include/trace/events/xen.h | 16 ----------------
 3 files changed, 2 insertions(+), 22 deletions(-)

diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index d33e7dbe3129..2d76106788a3 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -42,13 +42,11 @@ xmaddr_t arbitrary_virt_to_machine(void *vaddr)
 }
 EXPORT_SYMBOL_GPL(arbitrary_virt_to_machine);
 
-static void xen_flush_tlb_all(void)
+static noinline void xen_flush_tlb_all(void)
 {
 	struct mmuext_op *op;
 	struct multicall_space mcs;
 
-	trace_xen_mmu_flush_tlb_all(0);
-
 	preempt_disable();
 
 	mcs = xen_mc_entry(sizeof(*op));
diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 486c0a34d00b..2c30cabfda90 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -1310,13 +1310,11 @@ unsigned long xen_read_cr2_direct(void)
 	return this_cpu_read(xen_vcpu_info.arch.cr2);
 }
 
-static void xen_flush_tlb(void)
+static noinline void xen_flush_tlb(void)
 {
 	struct mmuext_op *op;
 	struct multicall_space mcs;
 
-	trace_xen_mmu_flush_tlb(0);
-
 	preempt_disable();
 
 	mcs = xen_mc_entry(sizeof(*op));
diff --git a/include/trace/events/xen.h b/include/trace/events/xen.h
index 7dd8f34c37df..fdcf88bcf0ea 100644
--- a/include/trace/events/xen.h
+++ b/include/trace/events/xen.h
@@ -352,22 +352,6 @@ DECLARE_EVENT_CLASS(xen_mmu_pgd,
 DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_pin);
 DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_unpin);
 
-TRACE_EVENT(xen_mmu_flush_tlb_all,
-	    TP_PROTO(int x),
-	    TP_ARGS(x),
-	    TP_STRUCT__entry(__array(char, x, 0)),
-	    TP_fast_assign((void)x),
-	    TP_printk("%s", "")
-	);
-
-TRACE_EVENT(xen_mmu_flush_tlb,
-	    TP_PROTO(int x),
-	    TP_ARGS(x),
-	    TP_STRUCT__entry(__array(char, x, 0)),
-	    TP_fast_assign((void)x),
-	    TP_printk("%s", "")
-	);
-
 TRACE_EVENT(xen_mmu_flush_tlb_one_user,
 	    TP_PROTO(unsigned long addr),
 	    TP_ARGS(addr),
-- 
2.13.6