linux-kernel - Re: [PATCH 00/35 v5] PTI support for x32

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Date:   Wed, 18 Apr 2018 09:35:58 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        "David H . Gutteridge" <dhgutteridge@...patico.ca>
Subject: Re: [PATCH 00/35 v5] PTI support for x32

On Mon, Apr 16, 2018 at 09:13:22AM -0700, Linus Torvalds wrote:
> See for example commit 8c06c7740d19 ("x86/pti: Leave kernel text
> global for !PCID") and in particular the performance numbers (that's
> an Atom microserver, but it was chosen due to lack of PCID).

Okay, I checked this on 32 bit and after some small changes I got
identical mappings with GLB set in all page-tables. The changes were:

	* Don't change permission bits in pti_clone_kernel_text().
	  Changing them does not make a difference on 64 bit as
	  everything cloned in this function is RO anyway. On 32 bit
	  some areas are mapped RW, so it does make a difference there.
	  
	  Having different permissions between kernel and user
	  page-table does also not make sense, because a permission
	  mismatch in the TLB will cause a re-walk, which is as fast as
	  not mapping it at all.

	* Mapping kernel-text to user-space on 32 bit too. Since there
	  is no PCID this should improve performance. I have not
	  measured that yet, but will do so before posting the next
	  version.

I do some more testing and performance measurements and will send
version 6 of my patches beginning of next week when v4.17-rc2 is out.


Regards,

	Joerg