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

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Message-ID: <CAK1hOcNU+8BtCN2TR6ZBh3KTTJtFKdoTe36UfVb91AYxEmeqTg@mail.gmail.com>
Date:	Mon, 9 Mar 2015 13:04:54 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Fengguang Wu <fengguang.wu@...el.com>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKP <lkp@...org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [x86/asm/entry] BUG: unable to handle kernel paging request

On Mon, Mar 9, 2015 at 12:46 PM, 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,

IIRC task gates weren't used in Linux at first, and now they are
only used for double fault handler - because in many cases
double fault is caused by stack problems, and task gate provides
a way to have alternative stack even for exceptions happening
in CPL0.
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