[PATCH v2 3/4] x86/fpu: Make the EFI FPU calling convention explicit

[Andy Lutomirski]

EFI uses kernel_fpu_begin() to conform to the UEFI calling convention.
This specifically requires initializing FCW, whereas no sane 64-bit kernel
code should use legacy 387 operations that reference FCW.

This should enable us to safely change the default semantics of
kernel_fpu_begin() to stop initializing FCW on 64-bit kernels.

Cc: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/efi.h     | 24 ++++++++++++++++++++----
 arch/x86/platform/efi/efi_64.c |  4 ++--
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
index bc9758ef292e..f3201544fbf8 100644
--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -68,17 +68,33 @@ extern unsigned long efi_fw_vendor, efi_config_table;
 		#f " called with too many arguments (" #p ">" #n ")");	\
 })
 
+static inline void efi_fpu_begin(void)
+{
+	/*
+	 * The UEFI calling convention (UEFI spec 2.3.2 and 2.3.4) requires
+	 * that FCW and MXCSR (64-bit) must be initialized prior to calling
+	 * UEFI code.  (Oddly the spec does not require that the FPU stack
+	 * be empty.)
+	 */
+	kernel_fpu_begin_mask(KFPU_387 | KFPU_MXCSR);
+}
+
+static inline void efi_fpu_end(void)
+{
+	kernel_fpu_end();
+}
+
 #ifdef CONFIG_X86_32
 #define arch_efi_call_virt_setup()					\
 ({									\
-	kernel_fpu_begin();						\
+	efi_fpu_begin();						\
 	firmware_restrict_branch_speculation_start();			\
 })
 
 #define arch_efi_call_virt_teardown()					\
 ({									\
 	firmware_restrict_branch_speculation_end();			\
-	kernel_fpu_end();						\
+	efi_fpu_end();							\
 })
 
 #define arch_efi_call_virt(p, f, args...)	p->f(args)
@@ -107,7 +123,7 @@ struct efi_scratch {
 #define arch_efi_call_virt_setup()					\
 ({									\
 	efi_sync_low_kernel_mappings();					\
-	kernel_fpu_begin();						\
+	efi_fpu_begin();						\
 	firmware_restrict_branch_speculation_start();			\
 	efi_switch_mm(&efi_mm);						\
 })
@@ -119,7 +135,7 @@ struct efi_scratch {
 ({									\
 	efi_switch_mm(efi_scratch.prev_mm);				\
 	firmware_restrict_branch_speculation_end();			\
-	kernel_fpu_end();						\
+	efi_fpu_end();							\
 })
 
 #ifdef CONFIG_KASAN
diff --git a/arch/x86/platform/efi/efi_64.c b/arch/x86/platform/efi/efi_64.c
index 8f5759df7776..f042040b5da1 100644
--- a/arch/x86/platform/efi/efi_64.c
+++ b/arch/x86/platform/efi/efi_64.c
@@ -848,7 +848,7 @@ efi_set_virtual_address_map(unsigned long memory_map_size,
 							 virtual_map);
 	efi_switch_mm(&efi_mm);
 
-	kernel_fpu_begin();
+	efi_fpu_begin();
 
 	/* Disable interrupts around EFI calls: */
 	local_irq_save(flags);
@@ -857,7 +857,7 @@ efi_set_virtual_address_map(unsigned long memory_map_size,
 			  descriptor_version, virtual_map);
 	local_irq_restore(flags);
 
-	kernel_fpu_end();
+	efi_fpu_end();
 
 	/* grab the virtually remapped EFI runtime services table pointer */
 	efi.runtime = READ_ONCE(systab->runtime);
-- 
2.29.2