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

Dave Hansen
2025-06-20T15:44:37Z

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.