linux-kernel - Re: [PATCHv6 05/16] x86/cpu: Defer CR pinning setup until after EFI initialization

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Message-ID: <f99b8106-d716-4c50-9906-34da5c8f4061@intel.com>
Date: Fri, 20 Jun 2025 08:44:33 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
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 "H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>,
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 Xiongwei Song <xiongwei.song@...driver.com>, Xin Li <xin3.li@...el.com>,
 "Mike Rapoport (IBM)" <rppt@...nel.org>,
 Brijesh Singh <brijesh.singh@....com>, Michael Roth <michael.roth@....com>,
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Cc: Jonathan Corbet <corbet@....net>, Sohil Mehta <sohil.mehta@...el.com>,
 Ingo Molnar <mingo@...nel.org>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
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Subject: Re: [PATCHv6 05/16] x86/cpu: Defer CR pinning setup until after EFI
 initialization

On 6/20/25 06:53, Kirill A. Shutemov wrote:
>  	/*
>  	 * This needs to follow the FPU initializtion, since EFI depends on it.
> +	 *
> +	 * It also needs to precede the CR pinning setup, because the CR4.LASS
> +	 * bit has to be cleared temporarily in order to execute the
> +	 * set_virtual_address_map() EFI call, which resides in lower addresses
> +	 * and would trip LASS if enabled.
> +	 *
> +	 * Wrapping efi_enter_virtual_mode() into lass_stac()/clac() is not
> +	 * enough because AC flag gates data accesses, but not instruction
> +	 * fetch. Clearing the CR4 bit is required.
>  	 */
>  	if (efi_enabled(EFI_RUNTIME_SERVICES))
>  		efi_enter_virtual_mode();
>  
> +	setup_cr_pinning();

This is a _bit_ too much of a comment for me in this context. This would
be enough:

	/* EFI twiddles CR4.LASS. Do it before CR pinning: */
  	if (efi_enabled(EFI_RUNTIME_SERVICES))
  		efi_enter_virtual_mode();

	setup_cr_pinning();

I also dislike comments where code will do. Could
efi_enter_virtual_mode() actually check for CR4.LASS? Or check
cr4_pinned_bits() or 'cr_pinning.key'?

Also, the "lass_stac()/clac() is not enough" comment seems more like
changelog material to me than something for a comment.