efi/x86: Limit EFI old memory map to SGI UV machines

We carry a quirk in the x86 EFI code to switch back to an older
method of mapping the EFI runtime services memory regions, because
it was deemed risky at the time to implement a new method without
providing a fallback to the old method in case problems arose.

Such problems did arise, but they appear to be limited to SGI UV1
machines, and so these are the only ones for which the fallback gets
enabled automatically (via a DMI quirk). The fallback can be enabled
manually as well, by passing efi=old_map, but there is very little
evidence that suggests that this is something that is being relied
upon in the field.

Given that UV1 support is not enabled by default by the distros
(Ubuntu, Fedora), there is no point in carrying this fallback code
all the time if there are no other users. So let's move it into the
UV support code, and document that efi=old_map now requires this
support code to be enabled.

Note that efi=old_map has been used in the past on other SGI UV
machines to work around kernel regressions in production, so we
keep the option to enable it by hand, but only if the kernel was
built with UV support.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200113172245.27925-8-ardb@kernel.org

1 files changed, 14 insertions, 12 deletions

diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
index 0a58468a7203..86169a24b0d8 100644
--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -20,13 +20,16 @@
  * This is the main reason why we're doing stable VA mappings for RT
  * services.
  *
- * This flag is used in conjunction with a chicken bit called
- * "efi=old_map" which can be used as a fallback to the old runtime
- * services mapping method in case there's some b0rkage with a
- * particular EFI implementation (haha, it is hard to hold up the
- * sarcasm here...).
+ * SGI UV1 machines are known to be incompatible with this scheme, so we
+ * provide an opt-out for these machines via a DMI quirk that sets the
+ * attribute below.
  */
-#define EFI_OLD_MEMMAP		EFI_ARCH_1
+#define EFI_UV1_MEMMAP         EFI_ARCH_1
+
+static inline bool efi_have_uv1_memmap(void)
+{
+	return IS_ENABLED(CONFIG_X86_UV) && efi_enabled(EFI_UV1_MEMMAP);
+}
 
 #define EFI32_LOADER_SIGNATURE	"EL32"
 #define EFI64_LOADER_SIGNATURE	"EL64"
@@ -119,7 +122,7 @@ struct efi_scratch {
 	kernel_fpu_begin();						\
 	firmware_restrict_branch_speculation_start();			\
 									\
-	if (!efi_enabled(EFI_OLD_MEMMAP))				\
+	if (!efi_have_uv1_memmap())					\
 		efi_switch_mm(&efi_mm);					\
 })
 
@@ -128,7 +131,7 @@ struct efi_scratch {
 
 #define arch_efi_call_virt_teardown()					\
 ({									\
-	if (!efi_enabled(EFI_OLD_MEMMAP))				\
+	if (!efi_have_uv1_memmap())					\
 		efi_switch_mm(efi_scratch.prev_mm);			\
 									\
 	firmware_restrict_branch_speculation_end();			\
@@ -172,6 +175,8 @@ extern void efi_delete_dummy_variable(void);
 extern void efi_switch_mm(struct mm_struct *mm);
 extern void efi_recover_from_page_fault(unsigned long phys_addr);
 extern void efi_free_boot_services(void);
+extern pgd_t * __init efi_uv1_memmap_phys_prolog(void);
+extern void __init efi_uv1_memmap_phys_epilog(pgd_t *save_pgd);
 
 struct efi_setup_data {
 	u64 fw_vendor;
@@ -203,10 +208,7 @@ static inline bool efi_runtime_supported(void)
 	if (IS_ENABLED(CONFIG_X86_64) == efi_enabled(EFI_64BIT))
 		return true;
 
-	if (IS_ENABLED(CONFIG_EFI_MIXED) && !efi_enabled(EFI_OLD_MEMMAP))
-		return true;
-
-	return false;
+	return IS_ENABLED(CONFIG_EFI_MIXED);
 }
 
 extern void parse_efi_setup(u64 phys_addr, u32 data_len);