linux-kernel - Re: [C/R ARM][PATCH 1/3] ARM: Rudimentary syscall interfaces

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Message-ID: <20100324140252.GC5704@count0.beaverton.ibm.com>
Date:	Wed, 24 Mar 2010 07:02:52 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
	containers <containers@...ts.linux-foundation.org>,
	Christoffer Dall <christofferdall@...istofferdall.dk>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...hat.com>
Subject: Re: [C/R ARM][PATCH 1/3] ARM: Rudimentary syscall interfaces

On Wed, Mar 24, 2010 at 12:57:46AM -0400, Oren Laadan wrote:
> On Tue, 23 Mar 2010, Matt Helsley wrote:
> 
> > On Tue, Mar 23, 2010 at 08:53:42PM +0000, Russell King - ARM Linux wrote:
> > > On Sun, Mar 21, 2010 at 09:06:03PM -0400, Christoffer Dall wrote:
> > > > This small commit introduces a global state of system calls for ARM
> > > > making it possible for a debugger or checkpointing to gain information
> > > > about another process' state with respect to system calls.
> > > 
> > > I don't particularly like the idea that we always store the syscall
> > > number to memory for every system call, whether the stored version is
> > > used or not.
> > > 
> > > Since ARM caches are generally not write allocate, this means mostly
> > > write-only variables can have a higher than expected expense.
> > > 
> > > Is there not some thread flag which can be checked to see if we need to
> > > store the syscall number?
> > 
> > Perhaps before we freeze the task we can save the syscall number on ARM.
> > The patches suggest that the signal delivery path -- which the freezer
> > utilizes -- has the syscall number already.
> > 
> > Should work since the threads must be frozen first anyway.
> 
> I like the idea.
> 
> However, would it also work for those cases when the freezing does not 
> occur from the signal delivery path - e.g. for vfork and ptraced tasks ?

We could just as easily set it before the vfork uninterruptible completion.
ptracing I'd don't know about though.

Cheers,
	-Matt Helsley
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