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Date: Fri, 16 May 2025 16:08:14 +0200
From: Ingo Molnar <mingo@kernel.org>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"H. Peter Anvin" <hpa@zytor.com>, Jonathan Corbet <corbet@lwn.net>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	Kieran Bingham <kbingham@kernel.org>,
	Michael Roth <michael.roth@amd.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	Sandipan Das <sandipan.das@amd.com>,
	Juergen Gross <jgross@suse.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-efi@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCHv3 2/4] x86/64/mm: Make SPARSEMEM_VMEMMAP the only memory
 model
Message-ID: <aCdGzpXSVx15gz90@gmail.com>
References: <20250516123306.3812286-1-kirill.shutemov@linux.intel.com>
 <20250516123306.3812286-3-kirill.shutemov@linux.intel.com>
 <30570ca0-8da4-4ebc-84d6-0a4badfb7154@intel.com>
 <rqkfqkkli57fbd5zkj3bwko44kmqqwnfdm766snm26y2so52ss@6it24qxv356q>
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* Kirill A. Shutemov <kirill.shutemov@linux.intel.com> wrote:

> On Fri, May 16, 2025 at 06:42:03AM -0700, Dave Hansen wrote:
> > On 5/16/25 05:33, Kirill A. Shutemov wrote:
> > > 5-level paging only supports SPARSEMEM_VMEMMAP. CONFIG_X86_5LEVEL is
> > > being phased out, making 5-level paging support mandatory.
> > > 
> > > Make CONFIG_SPARSEMEM_VMEMMAP mandatory for x86-64 and eliminate
> > > any associated conditional statements.
> > I think we have ourselves a catch-22 here.
> > 
> > SPARSEMEM_VMEMMAP was selected because the other sparsemem modes
> > couldn't handle a dynamic MAX_PHYS{MEM,ADDR}_BITS introduced by 5-level
> > paging. Now you're proposing making it static again, but keeping the
> > SPARSEMEM_VMEMMAP dependency.
> > 
> > If you remove the dynamic MAX_PHYS{MEM,ADDR}_BITS, you should also
> > remove the dependency on SPARSEMEM_VMEMMAP. No?
> 
> I guess. But how?
> 
> And is there any value to support !SPARSEMEM_VMEMMAP?

Not really IMHO:

  .config.opensuse.default:    CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
  .config.ubuntu.localinstall: CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
  .config.fedora.generic:      CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y
  .config.rhel.generic:        CONFIG_SPARSEMEM_VMEMMAP_ENABLE=y

Thanks,

	Ingo