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Subject[PATCH v2 3/4] x86/fpu: Make the EFI FPU calling convention explicit
Date
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@arndb.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
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
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