[PATCH v4 15/16] efi: Disable LASS around set_virtual_address_map call

[Alexander Shishkin]

Of all the EFI runtime services, set_virtual_address_map is the only one
that is called at its lower mapping, which LASS prohibits regardless of
EFLAGS.AC setting. The only way to allow this to happen is to disable
LASS in the CR4 register.

Disable LASS around this low address EFI call.

Signed-off-by: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
---
 arch/x86/platform/efi/efi.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
index f090ec972d7b..6e5c2bb4f4df 100644
--- a/arch/x86/platform/efi/efi.c
+++ b/arch/x86/platform/efi/efi.c
@@ -848,11 +848,24 @@ 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);
+
 	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);
+
 	if (status != EFI_SUCCESS) {
 		pr_err("Unable to switch EFI into virtual mode (status=%lx)!\n",
 		       status);
-- 
2.43.0