[PATCH 5/8] lguest: Remove CR0.TS support

[Andy Lutomirski]

Now that Linux never sets CR0.TS, lguest doesn't need to support it.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/lguest_hcall.h |  1 -
 arch/x86/lguest/boot.c              | 17 +++++++----------
 drivers/lguest/hypercalls.c         |  4 ----
 drivers/lguest/lg.h                 |  1 -
 drivers/lguest/x86/core.c           | 19 +------------------
 5 files changed, 8 insertions(+), 34 deletions(-)

diff --git a/arch/x86/include/asm/lguest_hcall.h b/arch/x86/include/asm/lguest_hcall.h
index ef01fef3eebc..6c119cfae218 100644
--- a/arch/x86/include/asm/lguest_hcall.h
+++ b/arch/x86/include/asm/lguest_hcall.h
@@ -9,7 +9,6 @@
 #define LHCALL_FLUSH_TLB	5
 #define LHCALL_LOAD_IDT_ENTRY	6
 #define LHCALL_SET_STACK	7
-#define LHCALL_TS		8
 #define LHCALL_SET_CLOCKEVENT	9
 #define LHCALL_HALT		10
 #define LHCALL_SET_PMD		13
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 25da5bc8d83d..d74afcdbc580 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -497,27 +497,24 @@ static void lguest_cpuid(unsigned int *ax, unsigned int *bx,
  * a whole series of functions like read_cr0() and write_cr0().
  *
  * We start with cr0.  cr0 allows you to turn on and off all kinds of basic
- * features, but Linux only really cares about one: the horrifically-named Task
- * Switched (TS) bit at bit 3 (ie. 8)
+ * features, but the only cr0 bit that Linux ever used at runtime was the
+ * horrifically-named Task Switched (TS) bit at bit 3 (ie. 8)
  *
  * What does the TS bit do?  Well, it causes the CPU to trap (interrupt 7) if
  * the floating point unit is used.  Which allows us to restore FPU state
- * lazily after a task switch, and Linux uses that gratefully, but wouldn't a
- * name like "FPUTRAP bit" be a little less cryptic?
+ * lazily after a task switch if we wanted to, but wouldn't a name like
+ * "FPUTRAP bit" be a little less cryptic?
  *
- * We store cr0 locally because the Host never changes it.  The Guest sometimes
- * wants to read it and we'd prefer not to bother the Host unnecessarily.
+ * Fortunately, Linux keeps it simple and doesn't use TS, so we can ignore
+ * cr0.
  */
-static unsigned long current_cr0;
 static void lguest_write_cr0(unsigned long val)
 {
-	lazy_hcall1(LHCALL_TS, val & X86_CR0_TS);
-	current_cr0 = val;
 }
 
 static unsigned long lguest_read_cr0(void)
 {
-	return current_cr0;
+	return 0;
 }
 
 /*
diff --git a/drivers/lguest/hypercalls.c b/drivers/lguest/hypercalls.c
index 19a32280731d..601f81c04873 100644
--- a/drivers/lguest/hypercalls.c
+++ b/drivers/lguest/hypercalls.c
@@ -109,10 +109,6 @@ static void do_hcall(struct lg_cpu *cpu, struct hcall_args *args)
 	case LHCALL_SET_CLOCKEVENT:
 		guest_set_clockevent(cpu, args->arg1);
 		break;
-	case LHCALL_TS:
-		/* This sets the TS flag, as we saw used in run_guest(). */
-		cpu->ts = args->arg1;
-		break;
 	case LHCALL_HALT:
 		/* Similarly, this sets the halted flag for run_guest(). */
 		cpu->halted = 1;
diff --git a/drivers/lguest/lg.h b/drivers/lguest/lg.h
index 69b3814afd2f..2356a2318034 100644
--- a/drivers/lguest/lg.h
+++ b/drivers/lguest/lg.h
@@ -43,7 +43,6 @@ struct lg_cpu {
 	struct mm_struct *mm; 	/* == tsk->mm, but that becomes NULL on exit */
 
 	u32 cr2;
-	int ts;
 	u32 esp1;
 	u16 ss1;
 
diff --git a/drivers/lguest/x86/core.c b/drivers/lguest/x86/core.c
index 6e9042e3d2a9..743253fc638f 100644
--- a/drivers/lguest/x86/core.c
+++ b/drivers/lguest/x86/core.c
@@ -247,14 +247,6 @@ unsigned long *lguest_arch_regptr(struct lg_cpu *cpu, size_t reg_off, bool any)
 void lguest_arch_run_guest(struct lg_cpu *cpu)
 {
 	/*
-	 * Remember the awfully-named TS bit?  If the Guest has asked to set it
-	 * we set it now, so we can trap and pass that trap to the Guest if it
-	 * uses the FPU.
-	 */
-	if (cpu->ts && fpregs_active())
-		stts();
-
-	/*
 	 * SYSENTER is an optimized way of doing system calls.  We can't allow
 	 * it because it always jumps to privilege level 0.  A normal Guest
 	 * won't try it because we don't advertise it in CPUID, but a malicious
@@ -282,10 +274,6 @@ void lguest_arch_run_guest(struct lg_cpu *cpu)
 	 if (boot_cpu_has(X86_FEATURE_SEP))
 		wrmsr(MSR_IA32_SYSENTER_CS, __KERNEL_CS, 0);
 
-	/* Clear the host TS bit if it was set above. */
-	if (cpu->ts && fpregs_active())
-		clts();
-
 	/*
 	 * If the Guest page faulted, then the cr2 register will tell us the
 	 * bad virtual address.  We have to grab this now, because once we
@@ -421,12 +409,7 @@ void lguest_arch_handle_trap(struct lg_cpu *cpu)
 			kill_guest(cpu, "Writing cr2");
 		break;
 	case 7: /* We've intercepted a Device Not Available fault. */
-		/*
-		 * If the Guest doesn't want to know, we already restored the
-		 * Floating Point Unit, so we just continue without telling it.
-		 */
-		if (!cpu->ts)
-			return;
+		/* No special handling is needed here. */
 		break;
 	case 32 ... 255:
 		/* This might be a syscall. */
-- 
2.7.4