[PATCH 20/24] x86/entry: Add the C version get_percpu_base()

[Lai Jiangshan]

From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>

It implements the C version of asm macro GET_PERCPU_BASE().

Not functional difference intended.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/entry/traps.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/arch/x86/entry/traps.c b/arch/x86/entry/traps.c
index e912bfbd2a61..9f4bc52410d0 100644
--- a/arch/x86/entry/traps.c
+++ b/arch/x86/entry/traps.c
@@ -945,6 +945,42 @@ struct pt_regs *do_error_entry(struct pt_regs *eregs)
 	/* Enter from kernel, don't move pt_regs */
 	return eregs;
 }
+
+#ifdef CONFIG_SMP
+/*
+ * CPU/node NR is loaded from the limit (size) field of a special segment
+ * descriptor entry in GDT.
+ *
+ * Do not use RDPID, because KVM loads guest's TSC_AUX on vm-entry and
+ * may not restore the host's value until the CPU returns to userspace.
+ * Thus the kernel would consume a guest's TSC_AUX if an NMI arrives
+ * while running KVM's run loop.
+ */
+static __always_inline unsigned int gdt_get_cpu(void)
+{
+	unsigned int p;
+
+	asm ("lsl %[seg],%[p]": [p] "=a" (p): [seg] "r" (__CPUNODE_SEG));
+
+	return p & VDSO_CPUNODE_MASK;
+}
+
+/*
+ * Fetch the per-CPU GSBASE value for this processor.
+ *
+ * We normally use %gs for accessing per-CPU data, but we are setting up
+ * %gs here and obviously can not use %gs itself to access per-CPU data.
+ */
+static __always_inline unsigned long get_percpu_base(void)
+{
+	return __per_cpu_offset[gdt_get_cpu()];
+}
+#else
+static __always_inline unsigned long get_percpu_base(void)
+{
+	return pcpu_unit_offsets;
+}
+#endif
 #endif
 
 static bool is_sysenter_singlestep(struct pt_regs *regs)
-- 
2.19.1.6.gb485710b