linux-kernel - Re: [Xen-devel] [PATCH 2/5] xen/x86-64: clean up warnings aboutIST-using traps

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Date:	Fri, 8 May 2009 10:58:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <jbeulich@...ell.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 2/5] xen/x86-64: clean up warnings
	aboutIST-using traps


* Jan Beulich <jbeulich@...ell.com> wrote:

> >>> Jeremy Fitzhardinge <jeremy@...p.org> 07.05.09 20:56 >>>
> >Ignore known IST-using traps.  Aside from the debugger traps, they're
> >low-level faults which Xen will handle for us, so the kernel needn't
> >worry about them.  Keep warning in case unknown trap starts using IST.
> >
> >Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> >---
> > arch/x86/xen/enlighten.c |   22 ++++++++++++++++++++--
> > 1 files changed, 20 insertions(+), 2 deletions(-)
> >
> >diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> >index cb49f57..88f3aa4 100644
> >--- a/arch/x86/xen/enlighten.c
> >+++ b/arch/x86/xen/enlighten.c
> >@@ -439,12 +439,30 @@ static int cvt_gate_to_trap(int vector, const gate_desc *val,
> > 
> > 	addr = gate_offset(*val);
> > #ifdef CONFIG_X86_64
> >+	/*
> >+	 * Look for known traps using IST, and substitute them
> >+	 * appropriately.  The debugger ones are the only ones we care
> >+	 * about.  Xen will handle faults like double_fault and
> >+	 * machine_check, so we should never see them.  Warn if
> >+	 * there's an unexpected IST-using fault handler.
> >+	 */
> > 	if (addr == (unsigned long)debug)
> > 		addr = (unsigned long)xen_debug;
> > 	else if (addr == (unsigned long)int3)
> > 		addr = (unsigned long)xen_int3;
> >-	else
> >-		WARN_ON(val->ist != 0);
> >+	else if (addr == (unsigned long)double_fault ||
> >+		 addr == (unsigned long)stack_segment) {
> 
> I don't think you want to exclude handling stack faults: Ordinary 
> memory references using rsp or rbp as the base register will cause 
> these instead of general protection faults when the resulting 
> effective address is non- canonical.

Yes. Also, patches 1-2-3 really just do the same thing, one fixes 
the other one. Would be nice to have a single patch for that whole 
ugly topic of 64-bit Xen not handling ISTs properly.

If this was a CPU we'd say "sorry, dont run Linux on it then" ...

I've applied the reservation fix to x86/urgent, and the #5 patch to 
x86/xen (it's more of a cleanup, not a fix for .30, right?).

	Ingo
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