linux-kernel - Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception

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Message-ID: <alpine.DEB.2.21.1904022027170.1812@nanos.tec.linutronix.de>
Date:   Tue, 2 Apr 2019 20:27:45 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [patch 15/14] x86/dumpstack/64: Speedup in_exception_stack()

On Tue, 2 Apr 2019, Andy Lutomirski wrote:
> > On Apr 2, 2019, at 9:48 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > 
> >> 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 than
> >>> + * PAGE_SIZE, all pages covering a particular stack will have the same
> >>> + * info.
> >>> + */
> >>> +static const struct estack_pages estack_pages[ESTACK_PAGES] ____cacheline_aligned = {
> >>> +    [CONDRANGE(DF)]        = ESTACK_PAGE(DOUBLEFAULT_IST, DF),
> >>> +    [CONDRANGE(NMI)]    = ESTACK_PAGE(NMI_IST, NMI),
> >>> +    [PAGERANGE(DB)]        = ESTACK_PAGE(DEBUG_IST, DB),
> >>> +    [CONDRANGE(MCE)]    = ESTACK_PAGE(MCE_IST, MCE),
> >> 
> >> 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).
> > 
> > Yes, lemme fix that up.
> > 
> >> 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.
> > 
> > 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 others and just have 4 of them (DB0, DB1, DB2, and DB3.  After all,
> overflowing from one debug stack into another is just as much of a bug as
> overflowing into a different IST stack.

That makes sense.

Thanks,

	tglx