linux-kernel - Re: [PATCHv9 04/16] x86/cpu: Defer CR pinning setup until core initcall

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Message-ID: <6e768f25-3a1c-48b9-bc53-56877a556a83@intel.com>
Date: Thu, 31 Jul 2025 17:01:37 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Sohil Mehta <sohil.mehta@...el.com>, Thomas Gleixner
 <tglx@...utronix.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 Kees Cook <kees@...nel.org>
Cc: Jonathan Corbet <corbet@....net>, Ingo Molnar <mingo@...nel.org>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 Kai Huang <kai.huang@...el.com>, Sandipan Das <sandipan.das@....com>,
 Breno Leitao <leitao@...ian.org>, Rick Edgecombe
 <rick.p.edgecombe@...el.com>, Alexei Starovoitov <ast@...nel.org>,
 Hou Tao <houtao1@...wei.com>, Juergen Gross <jgross@...e.com>,
 Vegard Nossum <vegard.nossum@...cle.com>, Eric Biggers
 <ebiggers@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>,
 "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Luis Chamberlain <mcgrof@...nel.org>, Yuntao Wang <ytcoode@...il.com>,
 Rasmus Villemoes <linux@...musvillemoes.dk>,
 Christophe Leroy <christophe.leroy@...roup.eu>, Tejun Heo <tj@...nel.org>,
 Changbin Du <changbin.du@...wei.com>,
 Huang Shijie <shijie@...amperecomputing.com>,
 Geert Uytterhoeven <geert+renesas@...der.be>,
 Namhyung Kim <namhyung@...nel.org>,
 Arnaldo Carvalho de Melo <acme@...hat.com>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, linux-mm@...ck.org,
 "Kirill A. Shutemov" <kas@...nel.org>,
 "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...hat.com>,
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 Tony Luck <tony.luck@...el.com>, Alexey Kardashevskiy <aik@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 X86-kernel <x86@...nel.org>
Subject: Re: [PATCHv9 04/16] x86/cpu: Defer CR pinning setup until core
 initcall

On 7/31/25 16:45, Sohil Mehta wrote:
> On 7/9/2025 10:00 AM, Dave Hansen wrote:
>> On 7/7/25 01:03, Kirill A. Shutemov wrote:
>>> Instead of moving setup_cr_pinning() below efi_enter_virtual_mode() in
>>> arch_cpu_finalize_init(), defer it until core initcall.
>> What are the side effects of this move? Are there other benefits? What
>> are the risks?
>>
> Picking this up from Kirill.. Reevaluating this, core_initcall() seems
> too late for setup_cr_pinning().
> 
> We need to have CR pinning completed, and the associated static key
> enabled before AP bring up. start_secondary()->cr4_init() depends on the
> cr_pinning static key to initialize CR4 for APs.

Sure, if you leave cr4_init() completely as-is.

'cr4_pinned_bits' should be set by the boot CPU. Secondary CPUs should
also read 'cr4_pinned_bits' when setting up their own cr4's,
unconditionally, independent of 'cr_pinning'.

The thing I think we should change is the pinning _enforcement_. The
easiest way to do that is to remove the static_branch_likely() in
cr4_init() and then delay flipping the static branch until just before
userspace starts.

Basically, split up the:

static void __init setup_cr_pinning(void)
{
    cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & cr4_pinned_mask;
    static_key_enable(&cr_pinning.key);
}

code into its two logical pieces:

 1. Populate 'cr4_pinned_bits' from the boot CPU so the secondaries can
    use it
 2. Enable the static key so pinning enforcement is enabled