1 files changed, 92 insertions, 0 deletions
diff --git a/arch/x86/kernel/fred.c b/arch/x86/kernel/fred.c
new file mode 100644
index 000000000000..5e2cd1004980
--- /dev/null
+++ b/arch/x86/kernel/fred.c
@@ -0,0 +1,92 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#include <linux/kernel.h>
+
+#include <asm/desc.h>
+#include <asm/fred.h>
+#include <asm/tlbflush.h>
+#include <asm/traps.h>
+
+/* #DB in the kernel would imply the use of a kernel debugger. */
+#define FRED_DB_STACK_LEVEL		1UL
+#define FRED_NMI_STACK_LEVEL		2UL
+#define FRED_MC_STACK_LEVEL		2UL
+/*
+ * #DF is the highest level because a #DF means "something went wrong
+ * *while delivering an exception*." The number of cases for which that
+ * can happen with FRED is drastically reduced and basically amounts to
+ * "the stack you pointed me to is broken." Thus, always change stacks
+ * on #DF, which means it should be at the highest level.
+ */
+#define FRED_DF_STACK_LEVEL		3UL
+
+#define FRED_STKLVL(vector, lvl)	((lvl) << (2 * (vector)))
+
+DEFINE_PER_CPU(unsigned long, fred_rsp0);
+EXPORT_PER_CPU_SYMBOL(fred_rsp0);
+
+void cpu_init_fred_exceptions(void)
+{
+	/* When FRED is enabled by default, remove this log message */
+	pr_info("Initialize FRED on CPU%d\n", smp_processor_id());
+
+	/*
+	 * If a kernel event is delivered before a CPU goes to user level for
+	 * the first time, its SS is NULL thus NULL is pushed into the SS field
+	 * of the FRED stack frame.  But before ERETS is executed, the CPU may
+	 * context switch to another task and go to user level.  Then when the
+	 * CPU comes back to kernel mode, SS is changed to __KERNEL_DS.  Later
+	 * when ERETS is executed to return from the kernel event handler, a #GP
+	 * fault is generated because SS doesn't match the SS saved in the FRED
+	 * stack frame.
+	 *
+	 * Initialize SS to __KERNEL_DS when enabling FRED to avoid such #GPs.
+	 */
+	loadsegment(ss, __KERNEL_DS);
+
+	wrmsrl(MSR_IA32_FRED_CONFIG,
+	       /* Reserve for CALL emulation */
+	       FRED_CONFIG_REDZONE |
+	       FRED_CONFIG_INT_STKLVL(0) |
+	       FRED_CONFIG_ENTRYPOINT(asm_fred_entrypoint_user));
+
+	wrmsrl(MSR_IA32_FRED_STKLVLS, 0);
+
+	/*
+	 * Ater a CPU offline/online cycle, the FRED RSP0 MSR should be
+	 * resynchronized with its per-CPU cache.
+	 */
+	wrmsrl(MSR_IA32_FRED_RSP0, __this_cpu_read(fred_rsp0));
+
+	wrmsrl(MSR_IA32_FRED_RSP1, 0);
+	wrmsrl(MSR_IA32_FRED_RSP2, 0);
+	wrmsrl(MSR_IA32_FRED_RSP3, 0);
+
+	/* Enable FRED */
+	cr4_set_bits(X86_CR4_FRED);
+	/* Any further IDT use is a bug */
+	idt_invalidate();
+
+	/* Use int $0x80 for 32-bit system calls in FRED mode */
+	setup_clear_cpu_cap(X86_FEATURE_SYSENTER32);
+	setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
+}
+
+/* Must be called after setup_cpu_entry_areas() */
+void cpu_init_fred_rsps(void)
+{
+	/*
+	 * The purpose of separate stacks for NMI, #DB and #MC *in the kernel*
+	 * (remember that user space faults are always taken on stack level 0)
+	 * is to avoid overflowing the kernel stack.
+	 */
+	wrmsrl(MSR_IA32_FRED_STKLVLS,
+	       FRED_STKLVL(X86_TRAP_DB,  FRED_DB_STACK_LEVEL) |
+	       FRED_STKLVL(X86_TRAP_NMI, FRED_NMI_STACK_LEVEL) |
+	       FRED_STKLVL(X86_TRAP_MC,  FRED_MC_STACK_LEVEL) |
+	       FRED_STKLVL(X86_TRAP_DF,  FRED_DF_STACK_LEVEL));
+
+	/* The FRED equivalents to IST stacks... */
+	wrmsrl(MSR_IA32_FRED_RSP1, __this_cpu_ist_top_va(DB));
+	wrmsrl(MSR_IA32_FRED_RSP2, __this_cpu_ist_top_va(NMI));
+	wrmsrl(MSR_IA32_FRED_RSP3, __this_cpu_ist_top_va(DF));
+}