Re: [Xen-devel] [PATCH v2 3/5] dbg_rw_guest_mem: Conditionally enable debug log output

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Thu, 9 Jan 2014 09:59:08 +0000
Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

> On Wed, 2014-01-08 at 16:38 -0800, Mukesh Rathor wrote:
> > On Wed, 8 Jan 2014 17:44:22 +0000
> > Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > 
> > > On Wed, 2014-01-08 at 18:04 +0100, Tim Deegan wrote:
> > > > At 16:47 +0000 on 08 Jan (1389196026), Ian Campbell wrote:
> > > > > On Wed, 2014-01-08 at 09:28 -0500, Don Slutz wrote:
> > > > > > > Using volatile is almost always wrong. Why do you think
> > > > > > > it is needed here?
> > > > > > 
> > > > > > This was from Mukesh Rathor:
> > > > > > 
> > > > > > http://lists.xen.org/archives/html/xen-devel/2014-01/msg00426.html
> > > > > > 
> > > > > > I saw no reason to make it volatile, but maybe "kdb" needs
> > > > > > this? Happy to change any way you want.
> > > > > 
> > > > > I'm not the maintainer but if I were I'd say drop the volatile
> > > > > and maybe mark it __read_mostly and perhaps __used too (since
> > > > > it's static it might otherwise get eliminated).
> > > > > 
> > > > > > > If anything this variable is exactly the opposite, i..e
> > > > > > > __read_mostly or even const (given that I can't see
> > > > > > > anything which writes it I suppose this is a compile time
> > > > > > > setting?)
> > > > > > 
> > > > > > That has been how I have been testing it so far (changing
> > > > > > the source to set values).  Mukesh claims to be able to
> > > > > > change it at will.  Not sure how const may effect this.
> > > > 
> > > > If the idea is to use kdb itself to frob the value, then it does
> > > > need something to stop the compiler caching it.  This might
> > > > even be one of the few cases where 'volatile' actually DTRT; it
> > > > would still be more in keeping with Xen style to use an
> > > > explicit read op (like atomic_read()) where the value is
> > > > consumed.
> > > 
> > > Is there any need to be asynchronously frobbing this value in the
> > > middle of a function within this file and expecting it to be
> > 
> > Yes. I can stop the machine via kdb or other debugger, change the
> > value during debug, and upon resuming it will start printing stuff.
> > Often this is needed when going thru several iterations of call
> > before problem is seen. Making it volatile makes sure the compiler
> > loads it every instance of its use. This is not in main path, only
> > debugger path, so the overhead should not be an issue.
> 
> So you want to be able to toggle the value in between two immediately
> adjacent debug print calls? While debugging the debugging
> infrastructure itself? (using itself?).

I often debug gdbsx via kdb, and one could use JTAG also to debug kdb,
or some other debugger. Debugging a debugger is hard. Since, the variable
is accessed via outside means that the compiler is not aware of, making
it volatile makes the most sense to me.

thanks,
Mukesh


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel