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

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Message-Id: <7789E14E-C623-4DB7-B076-76B931957C9C@amacapital.net>
Date:   Tue, 2 Apr 2019 10:11:19 -0600
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
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 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.