From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932465AbdJXMrs (ORCPT ); Tue, 24 Oct 2017 08:47:48 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:48164 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751965AbdJXMrp (ORCPT ); Tue, 24 Oct 2017 08:47:45 -0400 X-Google-Smtp-Source: ABhQp+Q+jg6Xw8yiv5kOL/AjesbiBWz9oCqXb0mD1o0zplnsP2WH6XWzrD9bqjQO4f5WVlTB58VdaQ== Date: Tue, 24 Oct 2017 14:47:41 +0200 From: Ingo Molnar To: "Kirill A. Shutemov" Cc: "Kirill A. Shutemov" , Ingo Molnar , Linus Torvalds , x86@kernel.org, Thomas Gleixner , "H. Peter Anvin" , Andrew Morton , Andy Lutomirski , Cyrill Gorcunov , Borislav Petkov , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/6] Boot-time switching between 4- and 5-level paging for 4.15, Part 1 Message-ID: <20171024124741.ux74rtbu2vqaf6zt@gmail.com> References: <20171020081853.lmnvaiydxhy5c63t@gmail.com> <20171020094152.skx5sh5ramq2a3vu@black.fi.intel.com> <20171020152346.f6tjybt7i5kzbhld@gmail.com> <20171020162349.3kwhdgv7qo45w4lh@node.shutemov.name> <20171023115658.geccs22o2t733np3@gmail.com> <20171023122159.wyztmsbgt5k2d4tb@node.shutemov.name> <20171023124014.mtklgmydspnvfcvg@gmail.com> <20171023124811.4i73242s5dotnn5k@node.shutemov.name> <20171024094039.4lonzocjt5kras7m@gmail.com> <20171024113819.pli7ifesp2u2rexi@node.shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171024113819.pli7ifesp2u2rexi@node.shutemov.name> User-Agent: NeoMutt/20170609 (1.8.3) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Kirill A. Shutemov wrote: > On Tue, Oct 24, 2017 at 11:40:40AM +0200, Ingo Molnar wrote: > > > > * Kirill A. Shutemov wrote: > > > > > On Mon, Oct 23, 2017 at 02:40:14PM +0200, Ingo Molnar wrote: > > > > > > > > * Kirill A. Shutemov wrote: > > > > > > > > > > Making a variable that 'looks' like a constant macro dynamic in a rare Kconfig > > > > > > scenario is asking for trouble. > > > > > > > > > > We expect boot-time page mode switching to be enabled in kernel of next > > > > > generation enterprise distros. It shoudn't be that rare. > > > > > > > > My point remains even with not-so-rare Kconfig dependency. > > > > > > I don't follow how introducing new variable that depends on Kconfig option > > > would help with the situation. > > > > A new, properly named variable or function (max_physmem_bits or > > max_physmem_bits()) that is not all uppercase would make it abundantly clear that > > it is not a constant but a runtime value. > > Would we need to rename every uppercase macros that would depend on > max_physmem_bits()? Like MAXMEM. MAXMEM isn't used in too many places either - what's the total impact of it? > > > We would end up with inverse situation: people would use MAX_PHYSMEM_BITS > > > where the new variable need to be used and we will in the same situation. > > > > It should result in sub-optimal resource allocations worst-case, right? > > I don't think it's the worst case. > > For instance, virt_addr_valid() depends indirectly on it: > > virt_addr_valid() > __virt_addr_valid() > phys_addr_valid() > boot_cpu_data.x86_phys_bits (initialized with MAX_PHYSMEM_BITS) > > virt_addr_valid() is used in things like implementation /dev/kmem. > > To me it's far more risky than occasional build breakage for > CONFIG_X86_5LEVEL=y. So why do we have two variables here, one boot_cpu_data.x86_phys_bits and the other MAX_PHYSMEM_BITS - both set once during boot? I'm trying to find a clean solution for this all - hiding a boot time dependency into a constant-looking value doesn't feel clean. Thanks, Ingo