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Subject: Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1904021746080.1676@nanos.tec.linutronix.de>
Date:   Tue, 2 Apr 2019 10:11:19 -0600
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>
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Message-Id: <7789E14E-C623-4DB7-B076-76B931957C9C@amacapital.net>
References: <20190331214020.836098943@linutronix.de> <20190331215136.039902969@linutronix.de> <alpine.DEB.2.21.1904021214340.1676@nanos.tec.linutronix.de> <20190402154329.scp7i7uqevubgwrz@treble> <alpine.DEB.2.21.1904021746080.1676@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Apr 2, 2019, at 9:48 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Tue, 2 Apr 2019, Josh Poimboeuf wrote:
>>> On Tue, Apr 02, 2019 at 12:19:46PM +0200, Thomas Gleixner wrote:
>>> +/*
>>> + * Array of exception stack page descriptors. If the stack is larger th=
an
>>> + * PAGE_SIZE, all pages covering a particular stack will have the same
>>> + * info.
>>> + */
>>> +static const struct estack_pages estack_pages[ESTACK_PAGES] ____cacheli=
ne_aligned =3D {
>>> +    [CONDRANGE(DF)]        =3D ESTACK_PAGE(DOUBLEFAULT_IST, DF),
>>> +    [CONDRANGE(NMI)]    =3D ESTACK_PAGE(NMI_IST, NMI),
>>> +    [PAGERANGE(DB)]        =3D ESTACK_PAGE(DEBUG_IST, DB),
>>> +    [CONDRANGE(MCE)]    =3D ESTACK_PAGE(MCE_IST, MCE),
>>=20
>> It would be nice if the *_IST macro naming aligned with the struct
>> cea_exception_stacks field naming.  Then you could just do, e.g.
>> ESTACKPAGE(DF).
>=20
> Yes, lemme fix that up.
>=20
>> Also it's a bit unfortunate that some of the stack size knowledge is
>> hard-coded here, i.e #DB always being > 1 page and non-#DB being
>> sometimes 1 page.
>=20
> The problem is that there is no way to make this macro maze conditional on=

> sizeof(). But my macro foo is rusty.

How about a much better fix: make the DB stack be the same size as all the o=
thers and just have 4 of them (DB0, DB1, DB2, and DB3.  After all, overflowi=
ng from one debug stack into another is just as much of a bug as overflowing=
 into a different IST stack.