path: root/arch/x86/kernel/fpu/core.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 *  Copyright (C) 1994 Linus Torvalds
 *
 *  Pentium III FXSR, SSE support
 *  General FPU state handling cleanups
 *	Gareth Hughes <gareth@valinux.com>, May 2000
 */
#include <asm/fpu/api.h>
#include <asm/fpu/regset.h>
#include <asm/fpu/sched.h>
#include <asm/fpu/signal.h>
#include <asm/fpu/types.h>
#include <asm/traps.h>
#include <asm/irq_regs.h>

#include <linux/hardirq.h>
#include <linux/pkeys.h>
#include <linux/vmalloc.h>

#include "context.h"
#include "internal.h"
#include "legacy.h"
#include "xstate.h"

#define CREATE_TRACE_POINTS
#include <asm/trace/fpu.h>

#ifdef CONFIG_X86_64
DEFINE_STATIC_KEY_FALSE(__fpu_state_size_dynamic);
DEFINE_PER_CPU(u64, xfd_state);
#endif

/* The FPU state configuration data for kernel and user space */
struct fpu_state_config	fpu_kernel_cfg __ro_after_init;
struct fpu_state_config fpu_user_cfg __ro_after_init;

/*
 * Represents the initial FPU state. It's mostly (but not completely) zeroes,
 * depending on the FPU hardware format:
 */
struct fpstate init_fpstate __ro_after_init;

/*
 * Track whether the kernel is using the FPU state
 * currently.
 *
 * This flag is used:
 *
 *   - by IRQ context code to potentially use the FPU
 *     if it's unused.
 *
 *   - to debug kernel_fpu_begin()/end() correctness
 */
static DEFINE_PER_CPU(bool, in_kernel_fpu);

/*
 * Track which context is using the FPU on the CPU:
 */
DEFINE_PER_CPU(struct fpu *, fpu_fpregs_owner_ctx);

static bool kernel_fpu_disabled(void)
{
	return this_cpu_read(in_kernel_fpu);
}

static bool interrupted_kernel_fpu_idle(void)
{
	return !kernel_fpu_disabled();
}

/*
 * Were we in user mode (or vm86 mode) when we were
 * interrupted?
 *
 * Doing kernel_fpu_begin/end() is ok if we are running
 * in an interrupt context from user mode - we'll just
 * save the FPU state as required.
 */
static bool interrupted_user_mode(void)
{
	struct pt_regs *regs = get_irq_regs();
	return regs && user_mode(regs);
}

/*
 * Can we use the FPU in kernel mode with the
 * whole "kernel_fpu_begin/end()" sequence?
 *
 * It's always ok in process context (ie "not interrupt")
 * but it is sometimes ok even from an irq.
 */
bool irq_fpu_usable(void)
{
	return !in_interrupt() ||
		interrupted_user_mode() ||
		interrupted_kernel_fpu_idle();
}
EXPORT_SYMBOL(irq_fpu_usable);

/*
 * Track AVX512 state use because it is known to slow the max clock
 * speed of the core.
 */
static void update_avx_timestamp(struct fpu *fpu)
{

#define AVX512_TRACKING_MASK	(XFEATURE_MASK_ZMM_Hi256 | XFEATURE_MASK_Hi16_ZMM)

	if (fpu->fpstate->regs.xsave.header.xfeatures & AVX512_TRACKING_MASK)
		fpu->avx512_timestamp = jiffies;
}

/*
 * Save the FPU register state in fpu->fpstate->regs. The register state is
 * preserved.
 *
 * Must be called with fpregs_lock() held.
 *
 * The legacy FNSAVE instruction clears all FPU state unconditionally, so
 * register state has to be reloaded. That might be a pointless exercise
 * when the FPU is going to be used by another task right after that. But
 * this only affects 20+ years old 32bit systems and avoids conditionals all
 * over the place.
 *
 * FXSAVE and all XSAVE variants preserve the FPU register state.
 */
void save_fpregs_to_fpstate(struct fpu *fpu)
{
	if (likely(use_xsave())) {
		os_xsave(fpu->fpstate);
		update_avx_timestamp(fpu);
		return;
	}

	if (likely(use_fxsr())) {
		fxsave(&fpu->fpstate->regs.fxsave);
		return;
	}

	/*
	 * Legacy FPU register saving, FNSAVE always clears FPU registers,
	 * so we have to reload them from the memory state.
	 */
	asm volatile("fnsave %[fp]; fwait" : [fp] "=m" (fpu->fpstate->regs.fsave));
	frstor(&fpu->fpstate->regs.fsave);
}

void restore_fpregs_from_fpstate(struct fpstate *fpstate, u64 mask)
{
	/*
	 * AMD K7/K8 and later CPUs up to Zen don't save/restore
	 * FDP/FIP/FOP unless an exception is pending. Clear the x87 state
	 * here by setting it to fixed values.  "m" is a random variable
	 * that should be in L1.
	 */
	if (unlikely(static_cpu_has_bug(X86_BUG_FXSAVE_LEAK))) {
		asm volatile(
			"fnclex\n\t"
			"emms\n\t"
			"fildl %P[addr]"	/* set F?P to defined value */
			: : [addr] "m" (fpstate));
	}

	if (use_xsave()) {
		/*
		 * Dynamically enabled features are enabled in XCR0, but
		 * usage requires also that the corresponding bits in XFD
		 * are cleared.  If the bits are set then using a related
		 * instruction will raise #NM. This allows to do the
		 * allocation of the larger FPU buffer lazy from #NM or if
		 * the task has no permission to kill it which would happen
		 * via #UD if the feature is disabled in XCR0.
		 *
		 * XFD state is following the same life time rules as
		 * XSTATE and to restore state correctly XFD has to be
		 * updated before XRSTORS otherwise the component would
		 * stay in or go into init state even if the bits are set
		 * in fpstate::regs::xsave::xfeatures.
		 */
		xfd_update_state(fpstate);

		/*
		 * Restoring state always needs to modify all features
		 * which are in @mask even if the current task cannot use
		 * extended features.
		 *
		 * So fpstate->xfeatures cannot be used here, because then
		 * a feature for which the task has no permission but was
		 * used by the previous task would not go into init state.
		 */
		mask = fpu_kernel_cfg.max_features & mask;

		os_xrstor(fpstate, mask);
	} else {
		if (use_fxsr())
			fxrstor(&fpstate->regs.fxsave);
		else
			frstor(&fpstate->regs.fsave);
	}
}

void fpu_reset_from_exception_fixup(void)
{
	restore_fpregs_from_fpstate(&init_fpstate, XFEATURE_MASK_FPSTATE);
}

#if IS_ENABLED(CONFIG_KVM)
static void __fpstate_reset(struct fpstate *fpstate);

bool fpu_alloc_guest_fpstate(struct fpu_guest *gfpu)
{
	struct fpstate *fpstate;
	unsigned int size;

	size = fpu_user_cfg.default_size + ALIGN(offsetof(struct fpstate, regs), 64);
	fpstate = vzalloc(size);
	if (!fpstate)
		return false;

	__fpstate_reset(fpstate);
	fpstate_init_user(fpstate);
	fpstate->is_valloc	= true;
	fpstate->is_guest	= true;

	gfpu->fpstate = fpstate;
	return true;
}
EXPORT_SYMBOL_GPL(fpu_alloc_guest_fpstate);

void fpu_free_guest_fpstate(struct fpu_guest *gfpu)
{
	struct fpstate *fps = gfpu->fpstate;

	if (!fps)
		return;

	if (WARN_ON_ONCE(!fps->is_valloc || !fps->is_guest || fps->in_use))
		return;

	gfpu->fpstate = NULL;
	vfree(fps);
}
EXPORT_SYMBOL_GPL(fpu_free_guest_fpstate);

int fpu_swap_kvm_fpstate(struct fpu_guest *guest_fpu, bool enter_guest)
{
	struct fpstate *guest_fps = guest_fpu->fpstate;
	struct fpu *fpu = &current->thread.fpu;
	struct fpstate *cur_fps = fpu->fpstate;

	fpregs_lock();
	if (!cur_fps->is_confidential && !test_thread_flag(TIF_NEED_FPU_LOAD))
		save_fpregs_to_fpstate(fpu);

	/* Swap fpstate */
	if (enter_guest) {
		fpu->__task_fpstate = cur_fps;
		fpu->fpstate = guest_fps;
		guest_fps->in_use = true;
	} else {
		guest_fps->in_use = false;
		fpu->fpstate = fpu->__task_fpstate;
		fpu->__task_fpstate = NULL;
	}

	cur_fps = fpu->fpstate;

	if (!cur_fps->is_confidential) {
		/* Includes XFD update */
		restore_fpregs_from_fpstate(cur_fps, XFEATURE_MASK_FPSTATE);
	} else {
		/*
		 * XSTATE is restored by firmware from encrypted
		 * memory. Make sure XFD state is correct while
		 * running with guest fpstate
		 */
		xfd_update_state(cur_fps);
	}

	fpregs_mark_activate();
	fpregs_unlock();
	return 0;
}
EXPORT_SYMBOL_GPL(fpu_swap_kvm_fpstate);

void fpu_copy_guest_fpstate_to_uabi(struct fpu_guest *gfpu, void *buf,
				    unsigned int size, u32 pkru)
{
	struct fpstate *kstate = gfpu->fpstate;
	union fpregs_state *ustate = buf;
	struct membuf mb = { .p = buf, .left = size };

	if (cpu_feature_enabled(X86_FEATURE_XSAVE)) {
		__copy_xstate_to_uabi_buf(mb, kstate, pkru, XSTATE_COPY_XSAVE);
	} else {
		memcpy(&ustate->fxsave, &kstate->regs.fxsave,
		       sizeof(ustate->fxsave));
		/* Make it restorable on a XSAVE enabled host */
		ustate->xsave.header.xfeatures = XFEATURE_MASK_FPSSE;
	}
}
EXPORT_SYMBOL_GPL(fpu_copy_guest_fpstate_to_uabi);

int fpu_copy_uabi_to_guest_fpstate(struct fpu_guest *gfpu, const void *buf,
				   u64 xcr0, u32 *vpkru)
{
	struct fpstate *kstate = gfpu->fpstate;
	const union fpregs_state *ustate = buf;
	struct pkru_state *xpkru;
	int ret;

	if (!cpu_feature_enabled(X86_FEATURE_XSAVE)) {
		if (ustate->xsave.header.xfeatures & ~XFEATURE_MASK_FPSSE)
			return -EINVAL;
		if (ustate->fxsave.mxcsr & ~mxcsr_feature_mask)
			return -EINVAL;
		memcpy(&kstate->regs.fxsave, &ustate->fxsave, sizeof(ustate->fxsave));
		return 0;
	}

	if (ustate->xsave.header.xfeatures & ~xcr0)
		return -EINVAL;

	ret = copy_uabi_from_kernel_to_xstate(kstate, ustate);
	if (ret)
		return ret;

	/* Retrieve PKRU if not in init state */
	if (kstate->regs.xsave.header.xfeatures & XFEATURE_MASK_PKRU) {
		xpkru = get_xsave_addr(&kstate->regs.xsave, XFEATURE_PKRU);
		*vpkru = xpkru->pkru;
	}

	/* Ensure that XCOMP_BV is set up for XSAVES */
	xstate_init_xcomp_bv(&kstate->regs.xsave, kstate->xfeatures);
	return 0;
}
EXPORT_SYMBOL_GPL(fpu_copy_uabi_to_guest_fpstate);
#endif /* CONFIG_KVM */

void kernel_fpu_begin_mask(unsigned int kfpu_mask)
{
	preempt_disable();

	WARN_ON_FPU(!irq_fpu_usable());
	WARN_ON_FPU(this_cpu_read(in_kernel_fpu));

	this_cpu_write(in_kernel_fpu, true);

	if (!(current->flags & PF_KTHREAD) &&
	    !test_thread_flag(TIF_NEED_FPU_LOAD)) {
		set_thread_flag(TIF_NEED_FPU_LOAD);
		save_fpregs_to_fpstate(&current->thread.fpu);
	}
	__cpu_invalidate_fpregs_state();

	/* Put sane initial values into the control registers. */
	if (likely(kfpu_mask & KFPU_MXCSR) && boot_cpu_has(X86_FEATURE_XMM))
		ldmxcsr(MXCSR_DEFAULT);

	if (unlikely(kfpu_mask & KFPU_387) && boot_cpu_has(X86_FEATURE_FPU))
		asm volatile ("fninit");
}
EXPORT_SYMBOL_GPL(kernel_fpu_begin_mask);

void kernel_fpu_end(void)
{
	WARN_ON_FPU(!this_cpu_read(in_kernel_fpu));

	this_cpu_write(in_kernel_fpu, false);
	preempt_enable();
}
EXPORT_SYMBOL_GPL(kernel_fpu_end);

/*
 * Sync the FPU register state to current's memory register state when the
 * current task owns the FPU. The hardware register state is preserved.
 */
void fpu_sync_fpstate(struct fpu *fpu)
{
	WARN_ON_FPU(fpu != &current->thread.fpu);

	fpregs_lock();
	trace_x86_fpu_before_save(fpu);

	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
		save_fpregs_to_fpstate(fpu);

	trace_x86_fpu_after_save(fpu);
	fpregs_unlock();
}

static inline unsigned int init_fpstate_copy_size(void)
{
	if (!use_xsave())
		return fpu_kernel_cfg.default_size;

	/* XSAVE(S) just needs the legacy and the xstate header part */
	return sizeof(init_fpstate.regs.xsave);
}

static inline void fpstate_init_fxstate(struct fpstate *fpstate)
{
	fpstate->regs.fxsave.cwd = 0x37f;
	fpstate->regs.fxsave.mxcsr = MXCSR_DEFAULT;
}

/*
 * Legacy x87 fpstate state init:
 */
static inline void fpstate_init_fstate(struct fpstate *fpstate)
{
	fpstate->regs.fsave.cwd = 0xffff037fu;
	fpstate->regs.fsave.swd = 0xffff0000u;
	fpstate->regs.fsave.twd = 0xffffffffu;
	fpstate->regs.fsave.fos = 0xffff0000u;
}

/*
 * Used in two places:
 * 1) Early boot to setup init_fpstate for non XSAVE systems
 * 2) fpu_init_fpstate_user() which is invoked from KVM
 */
void fpstate_init_user(struct fpstate *fpstate)
{
	if (!cpu_feature_enabled(X86_FEATURE_FPU)) {
		fpstate_init_soft(&fpstate->regs.soft);
		return;
	}

	xstate_init_xcomp_bv(&fpstate->regs.xsave, fpstate->xfeatures);

	if (cpu_feature_enabled(X86_FEATURE_FXSR))
		fpstate_init_fxstate(fpstate);
	else
		fpstate_init_fstate(fpstate);
}

static void __fpstate_reset(struct fpstate *fpstate)
{
	/* Initialize sizes and feature masks */
	fpstate->size		= fpu_kernel_cfg.default_size;
	fpstate->user_size	= fpu_user_cfg.default_size;
	fpstate->xfeatures	= fpu_kernel_cfg.default_features;
	fpstate->user_xfeatures	= fpu_user_cfg.default_features;
	fpstate->xfd		= init_fpstate.xfd;
}

void fpstate_reset(struct fpu *fpu)
{
	/* Set the fpstate pointer to the default fpstate */
	fpu->fpstate = &fpu->__fpstate;
	__fpstate_reset(fpu->fpstate);

	/* Initialize the permission related info in fpu */
	fpu->perm.__state_perm		= fpu_kernel_cfg.default_features;
	fpu->perm.__state_size		= fpu_kernel_cfg.default_size;
	fpu->perm.__user_state_size	= fpu_user_cfg.default_size;
}

static inline void fpu_inherit_perms(struct fpu *dst_fpu)
{
	if (fpu_state_size_dynamic()) {
		struct fpu *src_fpu = &current->group_leader->thread.fpu;

		spin_lock_irq(&current->sighand->siglock);
		/* Fork also inherits the permissions of the parent */
		dst_fpu->perm = src_fpu->perm;
		spin_unlock_irq(&current->sighand->siglock);
	}
}

/* Clone current's FPU state on fork */
int fpu_clone(struct task_struct *dst, unsigned long clone_flags)
{
	struct fpu *src_fpu = &current->thread.fpu;
	struct fpu *dst_fpu = &dst->thread.fpu;

	/* The new task's FPU state cannot be valid in the hardware. */
	dst_fpu->last_cpu = -1;

	fpstate_reset(dst_fpu);

	if (!cpu_feature_enabled(X86_FEATURE_FPU))
		return 0;

	/*
	 * Enforce reload for user space tasks and prevent kernel threads
	 * from trying to save the FPU registers on context switch.
	 */
	set_tsk_thread_flag(dst, TIF_NEED_FPU_LOAD);

	/*
	 * No FPU state inheritance for kernel threads and IO
	 * worker threads.
	 */
	if (dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
		/* Clear out the minimal state */
		memcpy(&dst_fpu->fpstate->regs, &init_fpstate.regs,
		       init_fpstate_copy_size());
		return 0;
	}

	/*
	 * If a new feature is added, ensure all dynamic features are
	 * caller-saved from here!
	 */
	BUILD_BUG_ON(XFEATURE_MASK_USER_DYNAMIC != XFEATURE_MASK_XTILE_DATA);

	/*
	 * Save the default portion of the current FPU state into the
	 * clone. Assume all dynamic features to be defined as caller-
	 * saved, which enables skipping both the expansion of fpstate
	 * and the copying of any dynamic state.
	 *
	 * Do not use memcpy() when TIF_NEED_FPU_LOAD is set because
	 * copying is not valid when current uses non-default states.
	 */
	fpregs_lock();
	if (test_thread_flag(TIF_NEED_FPU_LOAD))
		fpregs_restore_userregs();
	save_fpregs_to_fpstate(dst_fpu);
	if (!(clone_flags & CLONE_THREAD))
		fpu_inherit_perms(dst_fpu);
	fpregs_unlock();

	trace_x86_fpu_copy_src(src_fpu);
	trace_x86_fpu_copy_dst(dst_fpu);

	return 0;
}

/*
 * Whitelist the FPU register state embedded into task_struct for hardened
 * usercopy.
 */
void fpu_thread_struct_whitelist(unsigned long *offset, unsigned long *size)
{
	*offset = offsetof(struct thread_struct, fpu.__fpstate.regs);
	*size = fpu_kernel_cfg.default_size;
}

/*
 * Drops current FPU state: deactivates the fpregs and
 * the fpstate. NOTE: it still leaves previous contents
 * in the fpregs in the eager-FPU case.
 *
 * This function can be used in cases where we know that
 * a state-restore is coming: either an explicit one,
 * or a reschedule.
 */
void fpu__drop(struct fpu *fpu)
{
	preempt_disable();

	if (fpu == &current->thread.fpu) {
		/* Ignore delayed exceptions from user space */
		asm volatile("1: fwait\n"
			     "2:\n"
			     _ASM_EXTABLE(1b, 2b));
		fpregs_deactivate(fpu);
	}

	trace_x86_fpu_dropped(fpu);

	preempt_enable();
}

/*
 * Clear FPU registers by setting them up from the init fpstate.
 * Caller must do fpregs_[un]lock() around it.
 */
static inline void restore_fpregs_from_init_fpstate(u64 features_mask)
{
	if (use_xsave())
		os_xrstor(&init_fpstate, features_mask);
	else if (use_fxsr())
		fxrstor(&init_fpstate.regs.fxsave);
	else
		frstor(&init_fpstate.regs.fsave);

	pkru_write_default();
}

/*
 * Reset current->fpu memory state to the init values.
 */
static void fpu_reset_fpregs(void)
{
	struct fpu *fpu = &current->thread.fpu;

	fpregs_lock();
	fpu__drop(fpu);
	/*
	 * This does not change the actual hardware registers. It just
	 * resets the memory image and sets TIF_NEED_FPU_LOAD so a
	 * subsequent return to usermode will reload the registers from the
	 * task's memory image.
	 *
	 * Do not use fpstate_init() here. Just copy init_fpstate which has
	 * the correct content already except for PKRU.
	 *
	 * PKRU handling does not rely on the xstate when restoring for
	 * user space as PKRU is eagerly written in switch_to() and
	 * flush_thread().
	 */
	memcpy(&fpu->fpstate->regs, &init_fpstate.regs, init_fpstate_copy_size());
	set_thread_flag(TIF_NEED_FPU_LOAD);
	fpregs_unlock();
}

/*
 * Reset current's user FPU states to the init states.  current's
 * supervisor states, if any, are not modified by this function.  The
 * caller guarantees that the XSTATE header in memory is intact.
 */
void fpu__clear_user_states(struct fpu *fpu)
{
	WARN_ON_FPU(fpu != &current->thread.fpu);

	fpregs_lock();
	if (!cpu_feature_enabled(X86_FEATURE_FPU)) {
		fpu_reset_fpregs();
		fpregs_unlock();
		return;
	}

	/*
	 * Ensure that current's supervisor states are loaded into their
	 * corresponding registers.
	 */
	if (xfeatures_mask_supervisor() &&
	    !fpregs_state_valid(fpu, smp_processor_id()))
		os_xrstor_supervisor(fpu->fpstate);

	/* Reset user states in registers. */
	restore_fpregs_from_init_fpstate(XFEATURE_MASK_USER_RESTORE);

	/*
	 * Now all FPU registers have their desired values.  Inform the FPU
	 * state machine that current's FPU registers are in the hardware
	 * registers. The memory image does not need to be updated because
	 * any operation relying on it has to save the registers first when
	 * current's FPU is marked active.
	 */
	fpregs_mark_activate();
	fpregs_unlock();
}

void fpu_flush_thread(void)
{
	fpstate_reset(&current->thread.fpu);
	fpu_reset_fpregs();
}
/*
 * Load FPU context before returning to userspace.
 */
void switch_fpu_return(void)
{
	if (!static_cpu_has(X86_FEATURE_FPU))
		return;

	fpregs_restore_userregs();
}
EXPORT_SYMBOL_GPL(switch_fpu_return);

#ifdef CONFIG_X86_DEBUG_FPU
/*
 * If current FPU state according to its tracking (loaded FPU context on this
 * CPU) is not valid then we must have TIF_NEED_FPU_LOAD set so the context is
 * loaded on return to userland.
 */
void fpregs_assert_state_consistent(void)
{
	struct fpu *fpu = &current->thread.fpu;

	if (test_thread_flag(TIF_NEED_FPU_LOAD))
		return;

	WARN_ON_FPU(!fpregs_state_valid(fpu, smp_processor_id()));
}
EXPORT_SYMBOL_GPL(fpregs_assert_state_consistent);
#endif

void fpregs_mark_activate(void)
{
	struct fpu *fpu = &current->thread.fpu;

	fpregs_activate(fpu);
	fpu->last_cpu = smp_processor_id();
	clear_thread_flag(TIF_NEED_FPU_LOAD);
}

/*
 * x87 math exception handling:
 */

int fpu__exception_code(struct fpu *fpu, int trap_nr)
{
	int err;

	if (trap_nr == X86_TRAP_MF) {
		unsigned short cwd, swd;
		/*
		 * (~cwd & swd) will mask out exceptions that are not set to unmasked
		 * status.  0x3f is the exception bits in these regs, 0x200 is the
		 * C1 reg you need in case of a stack fault, 0x040 is the stack
		 * fault bit.  We should only be taking one exception at a time,
		 * so if this combination doesn't produce any single exception,
		 * then we have a bad program that isn't synchronizing its FPU usage
		 * and it will suffer the consequences since we won't be able to
		 * fully reproduce the context of the exception.
		 */
		if (boot_cpu_has(X86_FEATURE_FXSR)) {
			cwd = fpu->fpstate->regs.fxsave.cwd;
			swd = fpu->fpstate->regs.fxsave.swd;
		} else {
			cwd = (unsigned short)fpu->fpstate->regs.fsave.cwd;
			swd = (unsigned short)fpu->fpstate->regs.fsave.swd;
		}

		err = swd & ~cwd;
	} else {
		/*
		 * The SIMD FPU exceptions are handled a little differently, as there
		 * is only a single status/control register.  Thus, to determine which
		 * unmasked exception was caught we must mask the exception mask bits
		 * at 0x1f80, and then use these to mask the exception bits at 0x3f.
		 */
		unsigned short mxcsr = MXCSR_DEFAULT;

		if (boot_cpu_has(X86_FEATURE_XMM))
			mxcsr = fpu->fpstate->regs.fxsave.mxcsr;

		err = ~(mxcsr >> 7) & mxcsr;
	}

	if (err & 0x001) {	/* Invalid op */
		/*
		 * swd & 0x240 == 0x040: Stack Underflow
		 * swd & 0x240 == 0x240: Stack Overflow
		 * User must clear the SF bit (0x40) if set
		 */
		return FPE_FLTINV;
	} else if (err & 0x004) { /* Divide by Zero */
		return FPE_FLTDIV;
	} else if (err & 0x008) { /* Overflow */
		return FPE_FLTOVF;
	} else if (err & 0x012) { /* Denormal, Underflow */
		return FPE_FLTUND;
	} else if (err & 0x020) { /* Precision */
		return FPE_FLTRES;
	}

	/*
	 * If we're using IRQ 13, or supposedly even some trap
	 * X86_TRAP_MF implementations, it's possible
	 * we get a spurious trap, which is not an error.
	 */
	return 0;
}