linux-kernel - Re: [PATCH] x86: fix {pmd,pud}_{set,clear}

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Date:   Mon, 19 Feb 2018 10:04:15 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Juergen Gross" <jgross@...e.com>
Cc:     "Ingo Molnar" <mingo@...e.hu>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "lkml" <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: fix {pmd,pud}_{set,clear}_flags()

>>> On 19.02.18 at 17:23, <jgross@...e.com> wrote:
> On 19/02/18 15:48, Jan Beulich wrote:
>> Just like pte_{set,clear}_flags() their PMD and PUD counterparts should
>> not do any address translation. This was outright wrong under Xen
>> (causing a dead boot with no useful output on "suitable" systems), and
>> produced needlessly more complicated code (even if just slightly) when
>> paravirt was enabled.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>> Cc: stable@...r.kernel.org 
> 
> While the changes look alright I'm really curious which kind of failure
> you are seeing, especially considering how long those functions haven't
> been changed (since more than 6 years).

It's the new use of pmd_set_flags() in 2cc42bac1c ("x86-64/Xen:
eliminate W+X mappings"), and due to the (very odd afaic) identity
map handling the issue is invisible on most systems/setups.

Jan