linux-kernel - Re: [PATCH v5 05/16] x86/cpu: Defer CR pinning setup until after EFI initialization

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Message-ID: <a95a624b-214a-4b1f-ac16-dad5a9270bf8@intel.com>
Date: Tue, 29 Oct 2024 15:10:42 -0700
From: Sohil Mehta <sohil.mehta@...el.com>
To: Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Andy Lutomirski
	<luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar
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	<linux-efi@...r.kernel.org>
Subject: Re: [PATCH v5 05/16] x86/cpu: Defer CR pinning setup until after EFI
 initialization

On 10/28/2024 9:07 AM, Alexander Shishkin wrote:
> In order to map the EFI runtime services, set_virtual_address_map
> needs to be called, which resides in the lower half of the address
> space. This means that LASS needs to be temporarily disabled around
> this call. This can only be done before the CR pinning is set up.
> 

...

>  
>  	/*
>  	 * This needs to follow the FPU initializtion, since EFI depends on it.
> +	 * It also needs to precede the CR pinning setup, because we need to be
> +	 * able to temporarily clear the CR4.LASS bit in order to execute the
> +	 * set_virtual_address_map call, which resides in lower addresses and
> +	 * would trip LASS if enabled.
>  	 */

It would be helpful to describe why lass_stac()/clac() doesn't work here
and instead the heavy handed CR4 toggling is needed.

>  	if (efi_enabled(EFI_RUNTIME_SERVICES))
>  		efi_enter_virtual_mode();
>  
> +	setup_cr_pinning();
> +