linux-kernel - Re: [RFC][PATCH 01/12 v3] x86/kvm/tracing: Use helper function trace_seq_buffer

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Message-ID: <20141104143538.799e8716@gandalf.local.home>
Date:	Tue, 4 Nov 2014 14:35:38 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Rustad, Mark D" <mark.d.rustad@...el.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Kosina <jkosina@...e.cz>, Petr Mladek <pmladek@...e.cz>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC][PATCH 01/12 v3] x86/kvm/tracing: Use helper function
 trace_seq_buffer_ptr()

On Tue, 4 Nov 2014 14:09:54 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Tue, 4 Nov 2014 17:17:08 +0000
> "Rustad, Mark D" <mark.d.rustad@...el.com> wrote:
> 
> > On Nov 4, 2014, at 7:52 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > From: "Steven Rostedt (Red Hat)" <rostedt@...dmis.org>
> > > 
> > > To allow for the restructiong of the trace_seq code, we need users
> > > of it to use the helper functions instead of accessing the internals
> > > of the trace_seq structure itself.
> > > 
> > > Cc: Mark Rustad <mark.d.rustad@...el.com>
> > > Cc: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
> > > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> > > ---
> > > arch/x86/kvm/mmutrace.h | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/mmutrace.h b/arch/x86/kvm/mmutrace.h
> > > index 5aaf35641768..ce463a9cc8fb 100644
> > > --- a/arch/x86/kvm/mmutrace.h
> > > +++ b/arch/x86/kvm/mmutrace.h
> > > @@ -22,7 +22,7 @@
> > > 	__entry->unsync = sp->unsync;
> > > 
> > > #define KVM_MMU_PAGE_PRINTK() ({				        \
> > > -	const u32 saved_len = p->len;					\
> > > +	const char *saved_ptr = trace_seq_buffer_ptr(p);		\
> > 
> > I think the above should not be a const char *, because the location pointed to is surely being changed. It should either be a char * or a char * const.
> 
> Ah right. It should be 'char * const'.
> 

Actually, I take that back. The contents of saved_ptr should not be
modified.

It may seem strange, but the update is done via the trace_seq_printf().
Then that content is return back to the user. The user should
definitely *not* modify the contents of saved_ptr.

This patch is good as is. It should not be a char *, or char * const.

-- Steve


> 
> > 
> > > 	static const char *access_str[] = {			        \
> > > 		"---", "--x", "w--", "w-x", "-u-", "-ux", "wu-", "wux"  \
> > > 	};							        \
> > > @@ -41,7 +41,7 @@
> > > 			 role.nxe ? "" : "!",				\
> > > 			 __entry->root_count,				\
> > > 			 __entry->unsync ? "unsync" : "sync", 0);	\
> > > -	p->buffer + saved_len;						\
> > > +	saved_ptr;							\
> > > 		})
> > > 
> > > #define kvm_mmu_trace_pferr_flags       \
> > > -- 
> > > 2.1.1
> > 
> 

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