[PATCH] um: Enable preemption in UML

[anton.ivanov at cambridgegreys.com]

From: Anton Ivanov <anton.ivanov at cambridgegreys.com>

Preemption requires saving/restoring FPU state. This patch
adds support for it using GCC intrinsics.

Signed-off-by: Anton Ivanov <anton.ivanov at cambridgegreys.com>
---
 arch/um/Kconfig                         |  1 -
 arch/um/Makefile                        |  3 +-
 arch/um/include/asm/fpu/api.h           |  4 +-
 arch/um/include/asm/processor-generic.h |  1 +
 arch/um/kernel/Makefile                 |  2 +-
 arch/um/kernel/fpu.c                    | 49 +++++++++++++++++++++++++
 6 files changed, 55 insertions(+), 5 deletions(-)
 create mode 100644 arch/um/kernel/fpu.c

diff --git a/arch/um/Kconfig b/arch/um/Kconfig
index b5e179360534..603f5fd82293 100644
--- a/arch/um/Kconfig
+++ b/arch/um/Kconfig
@@ -11,7 +11,6 @@ config UML
 	select ARCH_HAS_KCOV
 	select ARCH_HAS_STRNCPY_FROM_USER
 	select ARCH_HAS_STRNLEN_USER
-	select ARCH_NO_PREEMPT
 	select HAVE_ARCH_AUDITSYSCALL
 	select HAVE_ARCH_KASAN if X86_64
 	select HAVE_ARCH_KASAN_VMALLOC if HAVE_ARCH_KASAN
diff --git a/arch/um/Makefile b/arch/um/Makefile
index 82f05f250634..35a059baadeb 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -61,7 +61,8 @@ KBUILD_CFLAGS += $(CFLAGS) $(CFLAGS-y) -D__arch_um__ \
 	$(ARCH_INCLUDE) $(MODE_INCLUDE) -Dvmap=kernel_vmap	\
 	-Dlongjmp=kernel_longjmp -Dsetjmp=kernel_setjmp \
 	-Din6addr_loopback=kernel_in6addr_loopback \
-	-Din6addr_any=kernel_in6addr_any -Dstrrchr=kernel_strrchr
+	-Din6addr_any=kernel_in6addr_any -Dstrrchr=kernel_strrchr \
+	-mxsave
 
 KBUILD_RUSTFLAGS += -Crelocation-model=pie
 
diff --git a/arch/um/include/asm/fpu/api.h b/arch/um/include/asm/fpu/api.h
index 71bfd9ef3938..0094624ae9b4 100644
--- a/arch/um/include/asm/fpu/api.h
+++ b/arch/um/include/asm/fpu/api.h
@@ -8,8 +8,8 @@
  * of x86 optimized copy, xor, etc routines into the
  * UML code tree. */
 
-#define kernel_fpu_begin() (void)0
-#define kernel_fpu_end() (void)0
+void kernel_fpu_begin(void);
+void kernel_fpu_end(void);
 
 static inline bool irq_fpu_usable(void)
 {
diff --git a/arch/um/include/asm/processor-generic.h b/arch/um/include/asm/processor-generic.h
index 7414154b8e9a..1c3287f1a444 100644
--- a/arch/um/include/asm/processor-generic.h
+++ b/arch/um/include/asm/processor-generic.h
@@ -44,6 +44,7 @@ struct thread_struct {
 			} cb;
 		} u;
 	} request;
+    u8 fpu[512] __aligned(16); /* Intel docs require xsave/xrestore area to be aligned to 16 bytes */
 };
 
 #define INIT_THREAD \
diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
index 811188be954c..5d9fbaa544be 100644
--- a/arch/um/kernel/Makefile
+++ b/arch/um/kernel/Makefile
@@ -16,7 +16,7 @@ extra-y := vmlinux.lds
 
 obj-y = config.o exec.o exitcode.o irq.o ksyms.o mem.o \
 	physmem.o process.o ptrace.o reboot.o sigio.o \
-	signal.o sysrq.o time.o tlb.o trap.o \
+	signal.o sysrq.o time.o tlb.o trap.o fpu.o\
 	um_arch.o umid.o maccess.o kmsg_dump.o capflags.o skas/
 obj-y += load_file.o
 
diff --git a/arch/um/kernel/fpu.c b/arch/um/kernel/fpu.c
new file mode 100644
index 000000000000..4142a08c474d
--- /dev/null
+++ b/arch/um/kernel/fpu.c
@@ -0,0 +1,49 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2023 Cambridge Greys Ltd
+ * Copyright (C) 2023 Red Hat Inc
+ */
+
+#include <linux/cpu.h>
+#include <linux/init.h>
+#include <asm/fpu/api.h>
+
+/*
+ * The critical section between kernel_fpu_begin() and kernel_fpu_end()
+ * is non-reentrant. It is the caller's responsibility to avoid reentrance.
+ */
+
+static DEFINE_PER_CPU(bool, in_kernel_fpu);
+
+void kernel_fpu_begin(void)
+{
+	preempt_disable();
+
+	WARN_ON(this_cpu_read(in_kernel_fpu));
+
+	this_cpu_write(in_kernel_fpu, true);
+
+#ifdef CONFIG_64BIT
+	__builtin_ia32_fxsave64(&current->thread.fpu);
+#else
+	__builtin_ia32_fxsave(&current->thread.fpu);
+#endif
+}
+
+EXPORT_SYMBOL_GPL(kernel_fpu_begin);
+
+void kernel_fpu_end(void)
+{
+	WARN_ON(!this_cpu_read(in_kernel_fpu));
+
+#ifdef CONFIG_64BIT
+	__builtin_ia32_fxrstor64(&current->thread.fpu);
+#else
+	__builtin_ia32_fxrstor(&current->thread.fpu);
+#endif
+	this_cpu_write(in_kernel_fpu, false);
+
+	preempt_enable();
+}
+EXPORT_SYMBOL_GPL(kernel_fpu_end);
+
-- 
2.30.2