x86/ldt: Disable 16-bit segments on Xen PV

Xen PV doesn't implement ESPFIX64, so they don't work right.  Disable
them.  Also print a warning the first time anyone tries to use a
16-bit segment on a Xen PV guest that would otherwise allow it
to help people diagnose this change in behavior.

This gets us closer to having all x86 selftests pass on Xen PV.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/92b2975459dfe5929ecf34c3896ad920bd9e3f2d.1593795633.git.luto@kernel.org

arch/x86/kernel/ldt.c

@@ -29,6 +29,8 @@
 #include <asm/mmu_context.h>
 #include <asm/pgtable_areas.h>
 
+#include <xen/xen.h>
+
 /* This is a multiple of PAGE_SIZE. */
 #define LDT_SLOT_STRIDE (LDT_ENTRIES * LDT_ENTRY_SIZE)
 
@@ -543,6 +545,37 @@ static int read_default_ldt(void __user *ptr, unsigned long bytecount)
 	return bytecount;
 }
 
+static bool allow_16bit_segments(void)
+{
+	if (!IS_ENABLED(CONFIG_X86_16BIT))
+		return false;
+
+#ifdef CONFIG_XEN_PV
+	/*
+	 * Xen PV does not implement ESPFIX64, which means that 16-bit
+	 * segments will not work correctly.  Until either Xen PV implements
+	 * ESPFIX64 and can signal this fact to the guest or unless someone
+	 * provides compelling evidence that allowing broken 16-bit segments
+	 * is worthwhile, disallow 16-bit segments under Xen PV.
+	 */
+	if (xen_pv_domain()) {
+		static DEFINE_MUTEX(xen_warning);
+		static bool warned;
+
+		mutex_lock(&xen_warning);
+		if (!warned) {
+			pr_info("Warning: 16-bit segments do not work correctly in a Xen PV guest\n");
+			warned = true;
+		}
+		mutex_unlock(&xen_warning);
+
+		return false;
+	}
+#endif
+
+	return true;
+}
+
 static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 {
 	struct mm_struct *mm = current->mm;
@@ -574,7 +607,7 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 		/* The user wants to clear the entry. */
 		memset(&ldt, 0, sizeof(ldt));
 	} else {
-		if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
+		if (!ldt_info.seg_32bit && !allow_16bit_segments()) {
 			error = -EINVAL;
 			goto out;
 		}