linux-kernel - Re: [x86/asm/entry] BUG: unable to handle kernel paging request

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Date:	Mon, 9 Mar 2015 06:15:54 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKP <lkp@...org>, Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [x86/asm/entry] BUG: unable to handle kernel paging request

On Mar 9, 2015 7:46 AM, "Ingo Molnar" <mingo@...nel.org> wrote:
>
>
> * Andy Lutomirski <luto@...capital.net> wrote:
>
> > >>      */
> > >>     unsigned long        stack[64];
> > >>
> > >> Last I checked, 0x100 != 64.  Also, wow, this is kind of disgusting. :)
> > >
> > >
> > > Seems to be unused: I commented it out on "defconfig" build
> > > and got no build errors.
> >
> > It's used.  On 32-bit, NMIs don't use task gates (I don't know why),
>
> So task gates were a sexy hardware acceleration feature, eons ago, and
> we used to rely on them a lot, but why should we use task gates for
> NMIs on modern CPUs?
>
> NMIs are performance critical for profiling, and task gates are more
> expensive than interrupt gates. Task gates are also an x86 anachronism
> whose performance might change negatively in the future. We don't use
> 32-bit segmentation tricks for similar reasons.
>
> The only place where we use task gates is the double fault handler,
> and we only do it because we must: if pagetables are screwed up then
> we really have to create a new context from scratch to be able to
> print debug info.

Makes sense.  I play with 32-bit stuff so rarely that I have very
little idea how it performs.

--Andy
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