Re: [PATCHv6 06/16] efi: Disable LASS around set_virtual_address_map() EFI call

[Dave Hansen]

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.