linux-kernel - Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be preempted

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Message-ID: <20150122153707.4625878b@gandalf.local.home>
Date:	Thu, 22 Jan 2015 15:37:07 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	kvm list <kvm@...r.kernel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to
 be preempted

On Thu, 22 Jan 2015 12:24:47 -0800
Andy Lutomirski <luto@...capital.net> wrote:

> > Also, please remove the "notrace", because function tracing goes an
> > extra step to not require RCU being visible. The only thing you get
> > with notrace is not being able to trace an otherwise traceable function.
> >
> 
> Is this also true for kprobes?  And can kprobes nest inside function
> tracing hooks?

No, kprobes are a bit more fragile than function tracing or tracepoints.

And nothing should nest inside a function hook (except for interrupts,
they are fine).

> 
> The other issue, above and beyond RCU, is that we can't let kprobes
> run on the int3 stack.  If Xen upcalls can happen when interrupts are
> off, then we may need this protection to prevent that type of
> recursion.  (This will be much less scary in 3.20, because userspace
> int3 instructions will no longer execute on the int3 stack.)

Does this execute between the start of the int3 interrupt handler and
the call of do_int3()?

-- Steve
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