From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <owner-linux-mm@kvack.org>
Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198])
	by kanga.kvack.org (Postfix) with ESMTP id 881B36B025F
	for <linux-mm@kvack.org>; Thu, 28 Sep 2017 10:28:13 -0400 (EDT)
Received: by mail-wr0-f198.google.com with SMTP id f51so2462255wrf.3
        for <linux-mm@kvack.org>; Thu, 28 Sep 2017 07:28:13 -0700 (PDT)
Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65])
        by mx.google.com with SMTPS id n23sor1060006edn.8.2017.09.28.07.28.11
        for <linux-mm@kvack.org>
        (Google Transport Security);
        Thu, 28 Sep 2017 07:28:12 -0700 (PDT)
Date: Thu, 28 Sep 2017 17:28:09 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCHv7 12/19] x86/mm: Adjust virtual address space layout in
 early boot.
Message-ID: <20170928142809.3ste5znjtjmucwig@node.shutemov.name>
References: <20170918105553.27914-1-kirill.shutemov@linux.intel.com>
 <20170918105553.27914-13-kirill.shutemov@linux.intel.com>
 <20170928083155.7qahecaeifz5em5f@gmail.com>
 <20170928132608.priml7nc7dmo5r6d@node.shutemov.name>
 <20170928133838.oa72tybvmyy3sfgw@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170928133838.oa72tybvmyy3sfgw@gmail.com>
Sender: owner-linux-mm@kvack.org
List-ID: <linux-mm.kvack.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Ingo Molnar <mingo@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>, x86@kernel.org, Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>, Andrew Morton <akpm@linux-foundation.org>, Andy Lutomirski <luto@amacapital.net>, Cyrill Gorcunov <gorcunov@openvz.org>, Borislav Petkov <bp@suse.de>, linux-mm@kvack.org, linux-kernel@vger.kernel.org

On Thu, Sep 28, 2017 at 03:38:38PM +0200, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> > On Thu, Sep 28, 2017 at 10:31:55AM +0200, Ingo Molnar wrote:
> > > 
> > > * Kirill A. Shutemov <kirill.shutemov@linux.intel.com> wrote:
> > > 
> > > > We need to adjust virtual address space to support switching between
> > > > paging modes.
> > > > 
> > > > The adjustment happens in __startup_64().
> > > 
> > > > +#ifdef CONFIG_X86_5LEVEL
> > > > +	if (__read_cr4() & X86_CR4_LA57) {
> > > > +		pgtable_l5_enabled = 1;
> > > > +		pgdir_shift = 48;
> > > > +		ptrs_per_p4d = 512;
> > > > +	}
> > > > +#endif
> > > 
> > > So CR4 really sucks as a parameter passing interface - was it us who enabled LA57 
> > > in the early boot code, right? Couldn't we add a flag which gets set there, or 
> > > something?
> > 
> > It's not necessary that we enabled LA57. At least I tried to write code
> > that doesn't assume this. We enable it if bootloader haven't done this
> > already for us.
> > 
> > What is so awful about using CR4 as passing interface? It's one-time
> > check, so performance shouldn't be an issue.
> 
> As a starter, this code is in generic x86 code [choose_random_location()], is this 
> CR4 bit known to AMD as well and is it guaranteed to be sane across all x86 CPUs? 
> I don't think so.

It's architectural thing, so it's consistent across all x86
implementations.

> CR4 is a poor interface to pass CPU features through. Generaly we try to enumerate 
> CPU features via CPUID, and/or enable synthetic CPU features in certain cases, and 
> work from there.

Okay, has_cpuflag(X86_FEATURE_LA57) seems would do.

-- 
 Kirill A. Shutemov

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>