linux-kernel - Re: [PATCHv6 06/16] efi: Disable LASS around set_virtual_address

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Message-ID: <5bb6275b-5046-4bda-8c92-434e59cac08f@intel.com>
Date: Fri, 20 Jun 2025 08:55:29 -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>,
 Ard Biesheuvel <ardb@...nel.org>, "Paul E. McKenney" <paulmck@...nel.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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 linux-kernel@...r.kernel.org, linux-efi@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCHv6 06/16] efi: Disable LASS around
 set_virtual_address_map() EFI call

On 6/20/25 06:53, Kirill A. Shutemov wrote:
> diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
> index 463b784499a8..94c335229697 100644
> --- a/arch/x86/platform/efi/efi.c
> +++ b/arch/x86/platform/efi/efi.c
> @@ -825,11 +825,24 @@ static void __init __efi_enter_virtual_mode(void)
>  
>  	efi_sync_low_kernel_mappings();
>  
> +	/*
> +	 * set_virtual_address_map is the only service located at lower
> +	 * addresses, so we have to temporarily disable LASS around it.
> +	 * Note that clearing EFLAGS.AC is not enough for this, the whole
> +	 * LASS needs to be disabled.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_LASS))
> +		cr4_clear_bits(X86_CR4_LASS);

Could we do it like this instead?

	unsigned long lass = cr4_read_shadow() & X86_FEATURE_LASS;
	...
	cr4_clear_bits(lass);

>  	status = efi_set_virtual_address_map(efi.memmap.desc_size * count,
>  					     efi.memmap.desc_size,
>  					     efi.memmap.desc_version,
>  					     (efi_memory_desc_t *)pa,
>  					     efi_systab_phys);
> +
> +	if (cpu_feature_enabled(X86_FEATURE_LASS))
> +		cr4_set_bits(X86_CR4_LASS);

and:

	cr4_set_bits(lass);

>  	if (status != EFI_SUCCESS) {
>  		pr_err("Unable to switch EFI into virtual mode (status=%lx)!\n",
>  		       status);

That way, neither the presence of X86_FEATURE_LASS nor the ordering of
setting up X86_CR4_LASS matters.

Let's say the CPU supports X86_FEATURE_LASS and this code gets called
before the kernel is ready for LASS. It would break as written above.