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

Kirill A. Shutemov
2025-06-20T17:50:37Z

On Fri, Jun 20, 2025 at 08:55:29AM -0700, Dave Hansen wrote:
> 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.

Okay, will rework this.

--
Kiryl Shutsemau / Kirill A. Shutemov