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

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Date:	Fri, 08 May 2009 08:20:07 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Ingo Molnar" <mingo@...e.hu>,
	"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

>>> 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.

>+		/* Don't need to handle these */
>+		return 0;
>+#ifdef CONFIG_X86_MCE
>+	} else if (addr == (unsigned long)machine_check) {
>+		return 0;
>+#endif
>+	} else {
>+		/* Some other trap using IST? */
>+		if (WARN_ON(val->ist != 0))
>+			return 0;
>+	}
> #endif	/* CONFIG_X86_64 */
> 	info->address = addr;
> 
>-- 
>1.6.0.6

Jan

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