linux-kernel - Re: [RFC] de-asmify the x86-64 system call slowpath

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Message-ID: <20140128164811.GH10323@ZenIV.linux.org.uk>
Date:	Tue, 28 Jan 2014 16:48:11 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Anvin <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Subject: Re: [RFC] de-asmify the x86-64 system call slowpath

On Tue, Jan 28, 2014 at 05:38:08PM +0100, Oleg Nesterov wrote:
> On 01/28, Al Viro wrote:
> >
> > On Mon, Jan 27, 2014 at 06:39:31PM +0100, Oleg Nesterov wrote:
> > > On 01/27, Al Viro wrote:
> > > >
> > > > Why is _TIF_UPROBE *not* a part
> > > > of _TIF_DO_NOTIFY_MASK, for example?
> > >
> > > Yes, please see another email. That is why uprobe_deny_signal()
> > > sets TIF_NOTIFY_RESUME along with TIF_UPROBE.
> >
> > *grumble* Can it end up modifying *regs?  From very cursory reading of
> > kernel/events/uprobe.c it seems to do so, so we probably want to leave
> > via iretq if that has hit, right?
> 
> But we do this anyway, restore_args path does iretq?
> 
> I mean, uprobe_notify_resume() is called from do_notify_resume(), it
> should be fine to modify*regs there?

See Linus' patch trying to avoid iretq path; it's really costly.  Looks
like that patch will have to treat _TIF_UPROBE the same way it treats
_TIF_SIGPENDING...
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