[RFC 20/30] x86/entry: Add C code for fast system call entries

[Andy Lutomirski]

This handles both SYSENTER and SYSCALL.  The asm glue will take care
of the differences.

Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
 arch/x86/entry/common.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 41d17508cf46..5725cdcec4de 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -24,6 +24,8 @@
 
 #include <asm/desc.h>
 #include <asm/traps.h>
+#include <asm/vdso.h>
+#include <asm/uaccess.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
@@ -360,4 +362,41 @@ __visible void do_int80_syscall_32(struct pt_regs *regs)
 
        syscall_return_slowpath(regs);
 }
+
+__visible void do_fast_syscall_32(struct pt_regs *regs)
+{
+       /*
+        * Called using the internal vDSO SYSENTER calling convention.
+        * Adjust regs so it looks like we entered using int80.
+        */
+
+       unsigned long landing_pad = (unsigned long)current->mm->context.vdso +
+               vdso_image_32.sym_int80_landing_pad;
+
+       /* SYSENTER loses EIP.  Fix it up. */
+       regs->ip = landing_pad;
+
+       /*
+        * Fetch ECX from where the vDSO stashed it.
+        *
+        * WARNING: We are in CONTEXT_USER and RCU isn't paying attention!
+        */
+       local_irq_enable();
+       if (get_user(*(u32 *)&regs->cx,
+                    (u32 __user __force *)(unsigned long)(u32)regs->sp)) {
+               /* User code screwed up. */
+               local_irq_disable();
+               regs->ax = -EFAULT;
+#ifdef CONFIG_CONTEXT_TRACKING
+               enter_from_user_mode();
+#endif
+               prepare_exit_to_usermode(regs);
+               return;
+       }
+       local_irq_disable();
+
+       /* Now this is just like a normal syscall. */
+       do_int80_syscall_32(regs);
+       return;
+}
 #endif
-- 
2.4.3

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