linux-kernel - Re: [patch 47/60] x86/ldt: Map LDT entries into fixmap

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Date:   Mon, 4 Dec 2017 23:51:00 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Rik van Riel <riel@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>
Subject: Re: [patch 47/60] x86/ldt: Map LDT entries into fixmap

On Mon, 4 Dec 2017, Andy Lutomirski wrote:

> On Mon, Dec 4, 2017 at 6:07 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > From: Thomas Gleixner <tglx@...utronix.de>
> >
> > LDT is not really commonly used on 64bit so the overhead of populating the
> > fixmap entries on context switch for the rare LDT syscall users is a
> > reasonable trade off vs. having extra dynamically managed mapping space per
> > process.
> >
> 
> Hmm, I wonder just how slow this is.  It might be okay.  It's
> certainly not the way I imagined it working.

I know, it was the laziest way I could come up with. The only nasty thing
here is that __set_fixmap() does a tlb flush which is pointless as that
happens anyway. On my todo list was a noflush variant for set_fixmap along
with a variant which takes a whole range. That would simplify other places
as well. Though with the plan to map that stuff to a different place we
actually can avoid the weirdness of fixmaps

Thanks,

	tglx