linux-kernel - Re: [PATCH 3/3] x86, extable: Handle early exceptions

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Message-ID: <20120420084647.GA3247@aftab.osrc.amd.com>
Date:	Fri, 20 Apr 2012 10:46:47 +0200
From:	Borislav Petkov <bp@...64.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Borislav Petkov <bp@...64.org>, "H. Peter Anvin" <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 3/3] x86, extable: Handle early exceptions

On Thu, Apr 19, 2012 at 11:11:00AM -0700, Linus Torvalds wrote:
> > OTOH, if we can share early and normal exception handling lookup code, then a
> > perfect hash would make sense as those exceptions would pile up.
> 
> Oh, any of this only makes sense if we can share it with the runtime
> exception lookup.
> 
> They are *seldom* hugely performance-critical, but there are some
> unusual loads where you do get a fair number of exceptions.

I was thinking, and this is probably purely hypothetical and tangential
to the topic but, what happens if we get an exception right in the
middle between using the early_idt_handler's and switching to the normal
trap handlers in trap_init()?

write_idt_entry() is a simple memcpy so what happens if we have a
half-written descriptor? Won't we need some locking there to atomize the
switch?

Or am I dreaming?

Thanks.

-- 
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Boris.

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