xstate.c

/*
 * xsave/xrstor support.
 *
 * Author: Suresh Siddha <suresh.b.siddha@intel.com>
 */
#include <linux/compat.h>
#include <linux/cpu.h>

#include <asm/fpu/api.h>
#include <asm/fpu/internal.h>
#include <asm/fpu/signal.h>
#include <asm/fpu/regset.h>

#include <asm/tlbflush.h>

static const char *xfeature_names[] =
{
	"x87 floating point registers"	,
	"SSE registers"			,
	"AVX registers"			,
	"MPX bounds registers"		,
	"MPX CSR"			,
	"AVX-512 opmask"		,
	"AVX-512 Hi256"			,
	"AVX-512 ZMM_Hi256"		,
	"unknown xstate feature"	,
};

/*
 * Mask of xstate features supported by the CPU and the kernel:
 */
u64 xfeatures_mask __read_mostly;

static unsigned int xstate_offsets[XFEATURE_MAX] = { [ 0 ... XFEATURE_MAX - 1] = -1};
static unsigned int xstate_sizes[XFEATURE_MAX]   = { [ 0 ... XFEATURE_MAX - 1] = -1};
static unsigned int xstate_comp_offsets[sizeof(xfeatures_mask)*8];

/*
 * Clear all of the X86_FEATURE_* bits that are unavailable
 * when the CPU has no XSAVE support.
 */
void fpu__xstate_clear_all_cpu_caps(void)
{
	setup_clear_cpu_cap(X86_FEATURE_XSAVE);
	setup_clear_cpu_cap(X86_FEATURE_XSAVEOPT);
	setup_clear_cpu_cap(X86_FEATURE_XSAVEC);
	setup_clear_cpu_cap(X86_FEATURE_XSAVES);
	setup_clear_cpu_cap(X86_FEATURE_AVX);
	setup_clear_cpu_cap(X86_FEATURE_AVX2);
	setup_clear_cpu_cap(X86_FEATURE_AVX512F);
	setup_clear_cpu_cap(X86_FEATURE_AVX512PF);
	setup_clear_cpu_cap(X86_FEATURE_AVX512ER);
	setup_clear_cpu_cap(X86_FEATURE_AVX512CD);
	setup_clear_cpu_cap(X86_FEATURE_MPX);
}

/*
 * Return whether the system supports a given xfeature.
 *
 * Also return the name of the (most advanced) feature that the caller requested:
 */
int cpu_has_xfeatures(u64 xfeatures_needed, const char **feature_name)
{
	u64 xfeatures_missing = xfeatures_needed & ~xfeatures_mask;

	if (unlikely(feature_name)) {
		long xfeature_idx, max_idx;
		u64 xfeatures_print;
		/*
		 * So we use FLS here to be able to print the most advanced
		 * feature that was requested but is missing. So if a driver
		 * asks about "XFEATURE_MASK_SSE | XFEATURE_MASK_YMM" we'll print the
		 * missing AVX feature - this is the most informative message
		 * to users:
		 */
		if (xfeatures_missing)
			xfeatures_print = xfeatures_missing;
		else
			xfeatures_print = xfeatures_needed;

		xfeature_idx = fls64(xfeatures_print)-1;
		max_idx = ARRAY_SIZE(xfeature_names)-1;
		xfeature_idx = min(xfeature_idx, max_idx);

		*feature_name = xfeature_names[xfeature_idx];
	}

	if (xfeatures_missing)
		return 0;

	return 1;
}
EXPORT_SYMBOL_GPL(cpu_has_xfeatures);

/*
 * When executing XSAVEOPT (or other optimized XSAVE instructions), if
 * a processor implementation detects that an FPU state component is still
 * (or is again) in its initialized state, it may clear the corresponding
 * bit in the header.xfeatures field, and can skip the writeout of registers
 * to the corresponding memory layout.
 *
 * This means that when the bit is zero, the state component might still contain
 * some previous - non-initialized register state.
 *
 * Before writing xstate information to user-space we sanitize those components,
 * to always ensure that the memory layout of a feature will be in the init state
 * if the corresponding header bit is zero. This is to ensure that user-space doesn't
 * see some stale state in the memory layout during signal handling, debugging etc.
 */
void fpstate_sanitize_xstate(struct fpu *fpu)
{
	struct fxregs_state *fx = &fpu->state.fxsave;
	int feature_bit;
	u64 xfeatures;

	if (!use_xsaveopt())
		return;

	xfeatures = fpu->state.xsave.header.xfeatures;

	/*
	 * None of the feature bits are in init state. So nothing else
	 * to do for us, as the memory layout is up to date.
	 */
	if ((xfeatures & xfeatures_mask) == xfeatures_mask)
		return;

	/*
	 * FP is in init state
	 */
	if (!(xfeatures & XFEATURE_MASK_FP)) {
		fx->cwd = 0x37f;
		fx->swd = 0;
		fx->twd = 0;
		fx->fop = 0;
		fx->rip = 0;
		fx->rdp = 0;
		memset(&fx->st_space[0], 0, 128);
	}

	/*
	 * SSE is in init state
	 */
	if (!(xfeatures & XFEATURE_MASK_SSE))
		memset(&fx->xmm_space[0], 0, 256);

	/*
	 * First two features are FPU and SSE, which above we handled
	 * in a special way already:
	 */
	feature_bit = 0x2;
	xfeatures = (xfeatures_mask & ~xfeatures) >> 2;

	/*
	 * Update all the remaining memory layouts according to their
	 * standard xstate layout, if their header bit is in the init
	 * state:
	 */
	while (xfeatures) {
		if (xfeatures & 0x1) {
			int offset = xstate_offsets[feature_bit];
			int size = xstate_sizes[feature_bit];

			memcpy((void *)fx + offset,
			       (void *)&init_fpstate.xsave + offset,
			       size);
		}

		xfeatures >>= 1;
		feature_bit++;
	}
}

/*
 * Enable the extended processor state save/restore feature.
 * Called once per CPU onlining.
 */
void fpu__init_cpu_xstate(void)
{
	if (!cpu_has_xsave || !xfeatures_mask)
		return;

	cr4_set_bits(X86_CR4_OSXSAVE);
	xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask);
}

/*
 * Note that in the future we will likely need a pair of
 * functions here: one for user xstates and the other for
 * system xstates.  For now, they are the same.
 */
static int xfeature_enabled(enum xfeature xfeature)
{
	return !!(xfeatures_mask & (1UL << xfeature));
}

/*
 * Record the offsets and sizes of various xstates contained
 * in the XSAVE state memory layout.
 */
static void __init setup_xstate_features(void)
{
	u32 eax, ebx, ecx, edx, i;
	/* start at the beginnning of the "extended state" */
	unsigned int last_good_offset = offsetof(struct xregs_state,
						 extended_state_area);

	for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
		if (!xfeature_enabled(i))
			continue;

		cpuid_count(XSTATE_CPUID, i, &eax, &ebx, &ecx, &edx);
		xstate_offsets[i] = ebx;
		xstate_sizes[i] = eax;
		/*
		 * In our xstate size checks, we assume that the
		 * highest-numbered xstate feature has the
		 * highest offset in the buffer.  Ensure it does.
		 */
		WARN_ONCE(last_good_offset > xstate_offsets[i],
			"x86/fpu: misordered xstate at %d\n", last_good_offset);
		last_good_offset = xstate_offsets[i];

		printk(KERN_INFO "x86/fpu: xstate_offset[%d]: %4d, xstate_sizes[%d]: %4d\n", i, ebx, i, eax);
	}
}

static void __init print_xstate_feature(u64 xstate_mask)
{
	const char *feature_name;

	if (cpu_has_xfeatures(xstate_mask, &feature_name))
		pr_info("x86/fpu: Supporting XSAVE feature 0x%02Lx: '%s'\n", xstate_mask, feature_name);
}

/*
 * Print out all the supported xstate features:
 */
static void __init print_xstate_features(void)
{
	print_xstate_feature(XFEATURE_MASK_FP);
	print_xstate_feature(XFEATURE_MASK_SSE);
	print_xstate_feature(XFEATURE_MASK_YMM);
	print_xstate_feature(XFEATURE_MASK_BNDREGS);
	print_xstate_feature(XFEATURE_MASK_BNDCSR);
	print_xstate_feature(XFEATURE_MASK_OPMASK);
	print_xstate_feature(XFEATURE_MASK_ZMM_Hi256);
	print_xstate_feature(XFEATURE_MASK_Hi16_ZMM);
}

/*
 * This function sets up offsets and sizes of all extended states in
 * xsave area. This supports both standard format and compacted format
 * of the xsave aread.
 */
static void __init setup_xstate_comp(void)
{
	unsigned int xstate_comp_sizes[sizeof(xfeatures_mask)*8];
	int i;

	/*
	 * The FP xstates and SSE xstates are legacy states. They are always
	 * in the fixed offsets in the xsave area in either compacted form
	 * or standard form.
	 */
	xstate_comp_offsets[0] = 0;
	xstate_comp_offsets[1] = offsetof(struct fxregs_state, xmm_space);

	if (!cpu_has_xsaves) {
		for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
			if (xfeature_enabled(i)) {
				xstate_comp_offsets[i] = xstate_offsets[i];
				xstate_comp_sizes[i] = xstate_sizes[i];
			}
		}
		return;
	}

	xstate_comp_offsets[FIRST_EXTENDED_XFEATURE] =
		FXSAVE_SIZE + XSAVE_HDR_SIZE;

	for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
		if (xfeature_enabled(i))
			xstate_comp_sizes[i] = xstate_sizes[i];
		else
			xstate_comp_sizes[i] = 0;

		if (i > FIRST_EXTENDED_XFEATURE)
			xstate_comp_offsets[i] = xstate_comp_offsets[i-1]
					+ xstate_comp_sizes[i-1];

	}
}

/*
 * setup the xstate image representing the init state
 */
static void __init setup_init_fpu_buf(void)
{
	static int on_boot_cpu = 1;

	WARN_ON_FPU(!on_boot_cpu);
	on_boot_cpu = 0;

	if (!cpu_has_xsave)
		return;

	setup_xstate_features();
	print_xstate_features();

	if (cpu_has_xsaves) {
		init_fpstate.xsave.header.xcomp_bv = (u64)1 << 63 | xfeatures_mask;
		init_fpstate.xsave.header.xfeatures = xfeatures_mask;
	}

	/*
	 * Init all the features state with header_bv being 0x0
	 */
	copy_kernel_to_xregs_booting(&init_fpstate.xsave);

	/*
	 * Dump the init state again. This is to identify the init state
	 * of any feature which is not represented by all zero's.
	 */
	copy_xregs_to_kernel_booting(&init_fpstate.xsave);
}

static int xfeature_is_supervisor(int xfeature_nr)
{
	/*
	 * We currently do not support supervisor states, but if
	 * we did, we could find out like this.
	 *
	 * SDM says: If state component i is a user state component,
	 * ECX[0] return 0; if state component i is a supervisor
	 * state component, ECX[0] returns 1.
	u32 eax, ebx, ecx, edx;
	cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx;
	return !!(ecx & 1);
	*/
	return 0;
}
/*
static int xfeature_is_user(int xfeature_nr)
{
	return !xfeature_is_supervisor(xfeature_nr);
}
*/

/*
 * This check is important because it is easy to get XSTATE_*
 * confused with XSTATE_BIT_*.
 */
#define CHECK_XFEATURE(nr) do {		\
	WARN_ON(nr < FIRST_EXTENDED_XFEATURE);	\
	WARN_ON(nr >= XFEATURE_MAX);	\
} while (0)

/*
 * We could cache this like xstate_size[], but we only use
 * it here, so it would be a waste of space.
 */
static int xfeature_is_aligned(int xfeature_nr)
{
	u32 eax, ebx, ecx, edx;

	CHECK_XFEATURE(xfeature_nr);
	cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
	/*
	 * The value returned by ECX[1] indicates the alignment
	 * of state component i when the compacted format
	 * of the extended region of an XSAVE area is used
	 */
	return !!(ecx & 2);
}

static int xfeature_uncompacted_offset(int xfeature_nr)
{
	u32 eax, ebx, ecx, edx;

	CHECK_XFEATURE(xfeature_nr);
	cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
	return ebx;
}

static int xfeature_size(int xfeature_nr)
{
	u32 eax, ebx, ecx, edx;

	CHECK_XFEATURE(xfeature_nr);
	cpuid_count(XSTATE_CPUID, xfeature_nr, &eax, &ebx, &ecx, &edx);
	return eax;
}

/*
 * 'XSAVES' implies two different things:
 * 1. saving of supervisor/system state
 * 2. using the compacted format
 *
 * Use this function when dealing with the compacted format so
 * that it is obvious which aspect of 'XSAVES' is being handled
 * by the calling code.
 */
static int using_compacted_format(void)
{
	return cpu_has_xsaves;
}

static void __xstate_dump_leaves(void)
{
	int i;
	u32 eax, ebx, ecx, edx;
	static int should_dump = 1;

	if (!should_dump)
		return;
	should_dump = 0;
	/*
	 * Dump out a few leaves past the ones that we support
	 * just in case there are some goodies up there
	 */
	for (i = 0; i < XFEATURE_MAX + 10; i++) {
		cpuid_count(XSTATE_CPUID, i, &eax, &ebx, &ecx, &edx);
		pr_warn("CPUID[%02x, %02x]: eax=%08x ebx=%08x ecx=%08x edx=%08x\n",
			XSTATE_CPUID, i, eax, ebx, ecx, edx);
	}
}

#define XSTATE_WARN_ON(x) do {							\
	if (WARN_ONCE(x, "XSAVE consistency problem, dumping leaves")) {	\
		__xstate_dump_leaves();						\
	}									\
} while (0)

#define XCHECK_SZ(sz, nr, nr_macro, __struct) do {			\
	if ((nr == nr_macro) &&						\
	    WARN_ONCE(sz != sizeof(__struct),				\
		"%s: struct is %zu bytes, cpu state %d bytes\n",	\
		__stringify(nr_macro), sizeof(__struct), sz)) {		\
		__xstate_dump_leaves();					\
	}								\
} while (0)

/*
 * We have a C struct for each 'xstate'.  We need to ensure
 * that our software representation matches what the CPU
 * tells us about the state's size.
 */
static void check_xstate_against_struct(int nr)
{
	/*
	 * Ask the CPU for the size of the state.
	 */
	int sz = xfeature_size(nr);
	/*
	 * Match each CPU state with the corresponding software
	 * structure.
	 */
	XCHECK_SZ(sz, nr, XFEATURE_YMM,       struct ymmh_struct);
	XCHECK_SZ(sz, nr, XFEATURE_BNDREGS,   struct mpx_bndreg_state);
	XCHECK_SZ(sz, nr, XFEATURE_BNDCSR,    struct mpx_bndcsr_state);
	XCHECK_SZ(sz, nr, XFEATURE_OPMASK,    struct avx_512_opmask_state);
	XCHECK_SZ(sz, nr, XFEATURE_ZMM_Hi256, struct avx_512_zmm_uppers_state);
	XCHECK_SZ(sz, nr, XFEATURE_Hi16_ZMM,  struct avx_512_hi16_state);

	/*
	 * Make *SURE* to add any feature numbers in below if
	 * there are "holes" in the xsave state component
	 * numbers.
	 */
	if ((nr < XFEATURE_YMM) ||
	    (nr >= XFEATURE_MAX)) {
		WARN_ONCE(1, "no structure for xstate: %d\n", nr);
		XSTATE_WARN_ON(1);
	}
}

/*
 * This essentially double-checks what the cpu told us about
 * how large the XSAVE buffer needs to be.  We are recalculating
 * it to be safe.
 */
static void do_extra_xstate_size_checks(void)
{
	int paranoid_xstate_size = FXSAVE_SIZE + XSAVE_HDR_SIZE;
	int i;

	for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
		if (!xfeature_enabled(i))
			continue;

		check_xstate_against_struct(i);
		/*
		 * Supervisor state components can be managed only by
		 * XSAVES, which is compacted-format only.
		 */
		if (!using_compacted_format())
			XSTATE_WARN_ON(xfeature_is_supervisor(i));

		/* Align from the end of the previous feature */
		if (xfeature_is_aligned(i))
			paranoid_xstate_size = ALIGN(paranoid_xstate_size, 64);
		/*
		 * The offset of a given state in the non-compacted
		 * format is given to us in a CPUID leaf.  We check
		 * them for being ordered (increasing offsets) in
		 * setup_xstate_features().
		 */
		if (!using_compacted_format())
			paranoid_xstate_size = xfeature_uncompacted_offset(i);
		/*
		 * The compacted-format offset always depends on where
		 * the previous state ended.
		 */
		paranoid_xstate_size += xfeature_size(i);
	}
	XSTATE_WARN_ON(paranoid_xstate_size != xstate_size);
}

/*
 * Calculate total size of enabled xstates in XCR0/xfeatures_mask.
 *
 * Note the SDM's wording here.  "sub-function 0" only enumerates
 * the size of the *user* states.  If we use it to size a buffer
 * that we use 'XSAVES' on, we could potentially overflow the
 * buffer because 'XSAVES' saves system states too.
 *
 * Note that we do not currently set any bits on IA32_XSS so
 * 'XCR0 | IA32_XSS == XCR0' for now.
 */
static unsigned int __init calculate_xstate_size(void)
{
	unsigned int eax, ebx, ecx, edx;
	unsigned int calculated_xstate_size;

	if (!cpu_has_xsaves) {
		/*
		 * - CPUID function 0DH, sub-function 0:
		 *    EBX enumerates the size (in bytes) required by
		 *    the XSAVE instruction for an XSAVE area
		 *    containing all the *user* state components
		 *    corresponding to bits currently set in XCR0.
		 */
		cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
		calculated_xstate_size = ebx;
	} else {
		/*
		 * - CPUID function 0DH, sub-function 1:
		 *    EBX enumerates the size (in bytes) required by
		 *    the XSAVES instruction for an XSAVE area
		 *    containing all the state components
		 *    corresponding to bits currently set in
		 *    XCR0 | IA32_XSS.
		 */
		cpuid_count(XSTATE_CPUID, 1, &eax, &ebx, &ecx, &edx);
		calculated_xstate_size = ebx;
	}
	return calculated_xstate_size;
}

/*
 * Will the runtime-enumerated 'xstate_size' fit in the init
 * task's statically-allocated buffer?
 */
static bool is_supported_xstate_size(unsigned int test_xstate_size)
{
	if (test_xstate_size <= sizeof(union fpregs_state))
		return true;

	pr_warn("x86/fpu: xstate buffer too small (%zu < %d), disabling xsave\n",
			sizeof(union fpregs_state), test_xstate_size);
	return false;
}

static int init_xstate_size(void)
{
	/* Recompute the context size for enabled features: */
	unsigned int possible_xstate_size = calculate_xstate_size();

	/* Ensure we have the space to store all enabled: */
	if (!is_supported_xstate_size(possible_xstate_size))
		return -EINVAL;

	/*
	 * The size is OK, we are definitely going to use xsave,
	 * make it known to the world that we need more space.
	 */
	xstate_size = possible_xstate_size;
	do_extra_xstate_size_checks();
	return 0;
}

/*
 * We enabled the XSAVE hardware, but something went wrong and
 * we can not use it.  Disable it.
 */
static void fpu__init_disable_system_xstate(void)
{
	xfeatures_mask = 0;
	cr4_clear_bits(X86_CR4_OSXSAVE);
	fpu__xstate_clear_all_cpu_caps();
}

/*
 * Enable and initialize the xsave feature.
 * Called once per system bootup.
 */
void __init fpu__init_system_xstate(void)
{
	unsigned int eax, ebx, ecx, edx;
	static int on_boot_cpu = 1;
	int err;

	WARN_ON_FPU(!on_boot_cpu);
	on_boot_cpu = 0;

	if (!cpu_has_xsave) {
		pr_info("x86/fpu: Legacy x87 FPU detected.\n");
		return;
	}

	if (boot_cpu_data.cpuid_level < XSTATE_CPUID) {
		WARN_ON_FPU(1);
		return;
	}

	cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
	xfeatures_mask = eax + ((u64)edx << 32);

	if ((xfeatures_mask & XFEATURE_MASK_FPSSE) != XFEATURE_MASK_FPSSE) {
		pr_err("x86/fpu: FP/SSE not present amongst the CPU's xstate features: 0x%llx.\n", xfeatures_mask);
		BUG();
	}

	/* Support only the state known to the OS: */
	xfeatures_mask = xfeatures_mask & XCNTXT_MASK;

	/* Enable xstate instructions to be able to continue with initialization: */
	fpu__init_cpu_xstate();
	err = init_xstate_size();
	if (err) {
		/* something went wrong, boot without any XSAVE support */
		fpu__init_disable_system_xstate();
		return;
	}

	update_regset_xstate_info(xstate_size, xfeatures_mask);
	fpu__init_prepare_fx_sw_frame();
	setup_init_fpu_buf();
	setup_xstate_comp();

	pr_info("x86/fpu: Enabled xstate features 0x%llx, context size is %d bytes, using '%s' format.\n",
		xfeatures_mask,
		xstate_size,
		cpu_has_xsaves ? "compacted" : "standard");
}

/*
 * Restore minimal FPU state after suspend:
 */
void fpu__resume_cpu(void)
{
	/*
	 * Restore XCR0 on xsave capable CPUs:
	 */
	if (cpu_has_xsave)
		xsetbv(XCR_XFEATURE_ENABLED_MASK, xfeatures_mask);
}

/*
 * Given the xsave area and a state inside, this function returns the
 * address of the state.
 *
 * This is the API that is called to get xstate address in either
 * standard format or compacted format of xsave area.
 *
 * Note that if there is no data for the field in the xsave buffer
 * this will return NULL.
 *
 * Inputs:
 *	xstate: the thread's storage area for all FPU data
 *	xstate_feature: state which is defined in xsave.h (e.g.
 *	XFEATURE_MASK_FP, XFEATURE_MASK_SSE, etc...)
 * Output:
 *	address of the state in the xsave area, or NULL if the
 *	field is not present in the xsave buffer.
 */
void *get_xsave_addr(struct xregs_state *xsave, int xstate_feature)
{
	int feature_nr = fls64(xstate_feature) - 1;
	/*
	 * Do we even *have* xsave state?
	 */
	if (!boot_cpu_has(X86_FEATURE_XSAVE))
		return NULL;

	/*
	 * We should not ever be requesting features that we
	 * have not enabled.  Remember that pcntxt_mask is
	 * what we write to the XCR0 register.
	 */
	WARN_ONCE(!(xfeatures_mask & xstate_feature),
		  "get of unsupported state");
	/*
	 * This assumes the last 'xsave*' instruction to
	 * have requested that 'xstate_feature' be saved.
	 * If it did not, we might be seeing and old value
	 * of the field in the buffer.
	 *
	 * This can happen because the last 'xsave' did not
	 * request that this feature be saved (unlikely)
	 * or because the "init optimization" caused it
	 * to not be saved.
	 */
	if (!(xsave->header.xfeatures & xstate_feature))
		return NULL;

	return (void *)xsave + xstate_comp_offsets[feature_nr];
}
EXPORT_SYMBOL_GPL(get_xsave_addr);

/*
 * This wraps up the common operations that need to occur when retrieving
 * data from xsave state.  It first ensures that the current task was
 * using the FPU and retrieves the data in to a buffer.  It then calculates
 * the offset of the requested field in the buffer.
 *
 * This function is safe to call whether the FPU is in use or not.
 *
 * Note that this only works on the current task.
 *
 * Inputs:
 *	@xsave_state: state which is defined in xsave.h (e.g. XFEATURE_MASK_FP,
 *	XFEATURE_MASK_SSE, etc...)
 * Output:
 *	address of the state in the xsave area or NULL if the state
 *	is not present or is in its 'init state'.
 */
const void *get_xsave_field_ptr(int xsave_state)
{
	struct fpu *fpu = &current->thread.fpu;

	if (!fpu->fpstate_active)
		return NULL;
	/*
	 * fpu__save() takes the CPU's xstate registers
	 * and saves them off to the 'fpu memory buffer.
	 */
	fpu__save(fpu);

	return get_xsave_addr(&fpu->state.xsave, xsave_state);
}