[RFC PATCH v4 5/5] arm64: neon: Allow EFI runtime services to use FPSIMD in irq context

[Dave Martin]

Now that kernel-mode NEON use is non-nestable and not allowed in
hardirq/nmi context, we have a problem if the kernel decides to
call into EFI during an interrupt that interrupted a
kernel_neon_begin()..._end() block.  This will occur if the kernel
tries to write diagnostic data to EFI persistent storage during
a panic triggered by an NMI for example.

EFI runtime services specify an ABI that clobbers the FPSIMD state,
rather than being able to use it optionally as an accelerator.
This means that EFI is really a special case and can be handled
separately.

To enable EFI calls from interrupts, this patch creates dedicated
__efi_fpsimd_{begin,end}() helpers solely for this purpose, which
save/restore to a separate percpu buffer if called in a context
where kernel_neon_begin() is not usable.

Signed-off-by: Dave Martin <Dave.Martin at arm.com>
---
 arch/arm64/include/asm/efi.h    |  5 +++--
 arch/arm64/include/asm/fpsimd.h |  4 ++++
 arch/arm64/kernel/fpsimd.c      | 49 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/efi.h b/arch/arm64/include/asm/efi.h
index 8f3043a..8358222 100644
--- a/arch/arm64/include/asm/efi.h
+++ b/arch/arm64/include/asm/efi.h
@@ -3,6 +3,7 @@
 
 #include <asm/boot.h>
 #include <asm/cpufeature.h>
+#include <asm/fpsimd.h>
 #include <asm/io.h>
 #include <asm/mmu_context.h>
 #include <asm/neon.h>
@@ -20,8 +21,8 @@ int efi_set_mapping_permissions(struct mm_struct *mm, efi_memory_desc_t *md);
 
 #define arch_efi_call_virt_setup()					\
 ({									\
-	kernel_neon_begin();						\
 	efi_virtmap_load();						\
+	__efi_fpsimd_begin();						\
 })
 
 #define arch_efi_call_virt(p, f, args...)				\
@@ -33,8 +34,8 @@ int efi_set_mapping_permissions(struct mm_struct *mm, efi_memory_desc_t *md);
 
 #define arch_efi_call_virt_teardown()					\
 ({									\
+	__efi_fpsimd_end();						\
 	efi_virtmap_unload();						\
-	kernel_neon_end();						\
 })
 
 #define ARCH_EFI_IRQ_FLAGS_MASK (PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h
index ff2f6cd..410c481 100644
--- a/arch/arm64/include/asm/fpsimd.h
+++ b/arch/arm64/include/asm/fpsimd.h
@@ -67,6 +67,10 @@ extern void fpsimd_update_current_state(struct fpsimd_state *state);
 
 extern void fpsimd_flush_task_state(struct task_struct *target);
 
+/* For use by EFI runtime services calls only */
+extern void __efi_fpsimd_begin(void);
+extern void __efi_fpsimd_end(void);
+
 #endif
 
 #endif
diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 14329d6..601063c 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -23,6 +23,7 @@
 #include <linux/kernel.h>
 #include <linux/init.h>
 #include <linux/percpu.h>
+#include <linux/preempt.h>
 #include <linux/sched/signal.h>
 #include <linux/signal.h>
 
@@ -319,6 +320,54 @@ void kernel_neon_end(void)
 }
 EXPORT_SYMBOL(kernel_neon_end);
 
+DEFINE_PER_CPU(struct fpsimd_state, efi_fpsimd_state);
+DEFINE_PER_CPU(bool, efi_fpsimd_state_used);
+
+/*
+ * EFI runtime services support functions
+ *
+ * The ABI for EFI runtime services allows EFI to use FPSIMD during the call.
+ * This means that for EFI (and only for EFI), we have to assume that FPSIMD
+ * is always used rather than being an optional accelerator.
+ *
+ * These functions provide the necessary support for ensuring FPSIMD
+ * save/restore in the contexts from which EFI is used.
+ *
+ * Do not use them for any other purpose -- if tempted to do so, you are
+ * either doing something wrong or you need to propose some refactoring.
+ */
+
+/*
+ * __efi_fpsimd_begin(): prepare FPSIMD for making an EFI runtime services call
+ */
+void __efi_fpsimd_begin(void)
+{
+	if (!system_supports_fpsimd())
+		return;
+
+	WARN_ON(preemptible());
+
+	if (may_use_simd()) {
+		kernel_neon_begin();
+		__this_cpu_write(efi_fpsimd_state_used, true);
+	} else
+		fpsimd_save_state(this_cpu_ptr(&efi_fpsimd_state));
+}
+
+/*
+ * __efi_fpsimd_end(): clean up FPSIMD after an EFI runtime services call
+ */
+void __efi_fpsimd_end(void)
+{
+	if (!system_supports_fpsimd())
+		return;
+
+	if (__this_cpu_xchg(efi_fpsimd_state_used, false))
+		kernel_neon_end();
+	else
+		fpsimd_load_state(this_cpu_ptr(&efi_fpsimd_state));
+}
+
 #endif /* CONFIG_KERNEL_MODE_NEON */
 
 #ifdef CONFIG_CPU_PM
-- 
2.1.4