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Linux 4.14, 4.19, 5.0+ compat: SIMD save/restore
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Contrary to initial testing we cannot rely on these kernels to
invalidate the per-cpu FPU state and restore the FPU registers.
Therefore, the kfpu_begin() and kfpu_end() functions have been
updated to unconditionally save and restore the FPU state.

Signed-off-by: Brian Behlendorf <[email protected]>
Issue #9346
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@behlendorf
behlendorf committed Oct 3, 2019
1 parent 2cc479d commit fd12869
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Showing 2 changed files with 143 additions and 81 deletions.
82 changes: 44 additions & 38 deletions config/kernel-fpu.m4
View file
Original file line number Diff line number Diff line change
Expand Up @@ -2,15 +2,9 @@ dnl #
dnl # Handle differences in kernel FPU code.
dnl #
dnl # Kernel
dnl # 5.2: The fpu->initialized flag was replaced by TIF_NEED_FPU_LOAD.
dnl # HAVE_KERNEL_TIF_NEED_FPU_LOAD
dnl #
dnl # 5.0: As an optimization SIMD operations performed by kernel
dnl # threads can skip saving and restoring their FPU context.
dnl # Wrappers have been introduced to determine the running
dnl # context and use either the SIMD or generic implementation.
dnl # 5.0: Wrappers have been introduced to save/restore the FPU state.
dnl # This change was made to the 4.19.38 and 4.14.120 LTS kernels.
dnl # HAVE_KERNEL_FPU_INITIALIZED
dnl # HAVE_KERNEL_FPU_INTERNAL
dnl #
dnl # 4.2: Use __kernel_fpu_{begin,end}()
dnl # HAVE_UNDERSCORE_KERNEL_FPU & KERNEL_EXPORTS_X86_FPU
Expand Down Expand Up @@ -61,22 +55,47 @@ AC_DEFUN([ZFS_AC_KERNEL_SRC_FPU], [
__kernel_fpu_end();
], [], [$ZFS_META_LICENSE])
ZFS_LINUX_TEST_SRC([fpu_initialized], [
#include <linux/module.h>
ZFS_LINUX_TEST_SRC([fpu_internal], [
#if defined(__x86_64) || defined(__x86_64__) || \
defined(__i386) || defined(__i386__)
#if !defined(__x86)
#define __x86
#endif
#endif
#if !defined(__x86)
#error Unsupported architecture
#endif
#include <linux/sched.h>
],[
struct fpu *fpu = &current->thread.fpu;
if (fpu->initialized) { return (0); };
])
ZFS_LINUX_TEST_SRC([tif_need_fpu_load], [
#include <linux/module.h>
#include <asm/thread_info.h>
#if !defined(PF_KTHREAD)
#error PF_KTHREAD not defined
#endif
#if !defined(TIF_NEED_FPU_LOAD)
#error "TIF_NEED_FPU_LOAD undefined"
#ifdef HAVE_KERNEL_FPU_API_HEADER
#include <asm/fpu/api.h>
#include <asm/fpu/internal.h>
#else
#include <asm/i387.h>
#include <asm/xcr.h>
#endif
#if !defined(XSTATE_XSAVE)
#error XSTATE_XSAVE not defined
#endif
#if !defined(XSTATE_XRESTORE)
#error XSTATE_XRESTORE not defined
#endif
],[])
],[
struct fpu *fpu = &current->thread.fpu;
union fpregs_state *st = &fpu->state;
struct fregs_state *fr __attribute__ ((unused)) = &st->fsave;
struct fxregs_state *fxr __attribute__ ((unused)) = &st->fxsave;
struct xregs_state *xr __attribute__ ((unused)) = &st->xsave;
fpu->last_cpu = -1;
])
])

AC_DEFUN([ZFS_AC_KERNEL_FPU], [
Expand Down Expand Up @@ -104,25 +123,12 @@ AC_DEFUN([ZFS_AC_KERNEL_FPU], [
AC_DEFINE(KERNEL_EXPORTS_X86_FPU, 1,
[kernel exports FPU functions])
],[
dnl #
dnl # Linux 5.0 kernel
dnl #
ZFS_LINUX_TEST_RESULT([fpu_initialized], [
AC_MSG_RESULT(fpu.initialized)
AC_DEFINE(HAVE_KERNEL_FPU_INITIALIZED, 1,
[kernel fpu.initialized exists])
ZFS_LINUX_TEST_RESULT([fpu_internal], [
AC_MSG_RESULT(internal)
AC_DEFINE(HAVE_KERNEL_FPU_INTERNAL, 1,
[kernel fpu internal])
],[
dnl #
dnl # Linux 5.2 kernel
dnl #
ZFS_LINUX_TEST_RESULT([tif_need_fpu_load], [
AC_MSG_RESULT(TIF_NEED_FPU_LOAD)
AC_DEFINE(
HAVE_KERNEL_TIF_NEED_FPU_LOAD, 1,
[kernel TIF_NEED_FPU_LOAD exists])
],[
AC_MSG_RESULT(unavailable)
])
AC_MSG_RESULT(unavailable)
])
])
])
Expand Down
142 changes: 99 additions & 43 deletions include/os/linux/kernel/linux/simd_x86.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -126,38 +126,64 @@
#endif

#else /* defined(KERNEL_EXPORTS_X86_FPU) */

/*
* When the kernel_fpu_* symbols are unavailable then provide our own
* versions which allow the FPU to be safely used in kernel threads.
* In practice, this is not a significant restriction for ZFS since the
* vast majority of SIMD operations are performed by the IO pipeline.
*/
#if defined(HAVE_KERNEL_FPU_INTERNAL)

/*
* Returns non-zero if FPU operations are allowed in the current context.
* FPU usage only allowed in dedicated kernel threads.
*/
#if defined(HAVE_KERNEL_TIF_NEED_FPU_LOAD)
#define kfpu_allowed() ((current->flags & PF_KTHREAD) && \
test_thread_flag(TIF_NEED_FPU_LOAD))
#elif defined(HAVE_KERNEL_FPU_INITIALIZED)
#define kfpu_allowed() ((current->flags & PF_KTHREAD) && \
current->thread.fpu.initialized)
#else
#define kfpu_allowed() 0
#endif
#define kfpu_allowed() (current->flags & PF_KTHREAD)
#define ex_handler_fprestore ex_handler_default

/*
* FPU save and restore instructions.
*/
#define __asm __asm__ __volatile__
#define kfpu_fxsave(addr) __asm("fxsave %0" : "=m" (*(addr)))
#define kfpu_fxsaveq(addr) __asm("fxsaveq %0" : "=m" (*(addr)))
#define kfpu_fnsave(addr) __asm("fnsave %0" : "=m" (*(addr)))
#define kfpu_fxrstor(addr) __asm("fxrstor %0" : : "m" (*(addr)))
#define kfpu_fxrstorq(addr) __asm("fxrstorq %0" : : "m" (*(addr)))
#define kfpu_frstor(addr) __asm("frstor %0" : : "m" (*(addr)))

static inline void
kfpu_initialize(void)
{
WARN_ON_ONCE(!(current->flags & PF_KTHREAD));
current->thread.fpu.last_cpu = -1;
}

#if defined(HAVE_KERNEL_TIF_NEED_FPU_LOAD)
__fpu_invalidate_fpregs_state(&current->thread.fpu);
set_thread_flag(TIF_NEED_FPU_LOAD);
#elif defined(HAVE_KERNEL_FPU_INITIALIZED)
__fpu_invalidate_fpregs_state(&current->thread.fpu);
current->thread.fpu.initialized = 1;
#endif
static inline void
kfpu_save_xsave(struct xregs_state *addr, uint64_t mask)
{
uint32_t low, hi;
int err;

low = mask;
hi = mask >> 32;
XSTATE_XSAVE(addr, low, hi, err);
WARN_ON_ONCE(err);
}

static inline void
kfpu_save_fxsr(struct fxregs_state *addr)
{
if (IS_ENABLED(CONFIG_X86_32))
kfpu_fxsave(addr);
else
kfpu_fxsaveq(addr);
}

static inline void
kfpu_save_fsave(struct fregs_state *addr)
{
kfpu_fnsave(addr);
}

static inline void
Expand All @@ -172,46 +198,76 @@ kfpu_begin(void)
preempt_disable();
local_irq_disable();

#if defined(HAVE_KERNEL_TIF_NEED_FPU_LOAD)
/*
* The current FPU registers need to be preserved by kfpu_begin()
* and restored by kfpu_end(). This is required because we can
* not call __cpu_invalidate_fpregs_state() to invalidate the
* per-cpu FPU state and force them to be restored during a
* context switch.
*/
copy_fpregs_to_fpstate(&current->thread.fpu);
#elif defined(HAVE_KERNEL_FPU_INITIALIZED)
/*
* There is no need to preserve and restore the FPU registers.
* They will always be restored from the task's stored FPU state
* when switching contexts.
* and restored by kfpu_end(). This is always required because we
* can not call __cpu_invalidate_fpregs_state() to invalidate the
* per-cpu FPU state and force them to be restored. Furthermore,
* this can only be done safely in a kernel thread since by
* definition the task can never contain stored user FPU state.
*/
WARN_ON_ONCE(current->thread.fpu.initialized == 0);
#endif
if (static_cpu_has(X86_FEATURE_XSAVE)) {
kfpu_save_xsave(&current->thread.fpu.state.xsave, ~0);
} else if (static_cpu_has(X86_FEATURE_FXSR)) {
kfpu_save_fxsr(&current->thread.fpu.state.fxsave);
} else {
kfpu_save_fsave(&current->thread.fpu.state.fsave);
}
}

static inline void
kfpu_restore_xsave(struct xregs_state *addr, uint64_t mask)
{
uint32_t low, hi;

low = mask;
hi = mask >> 32;
XSTATE_XRESTORE(addr, low, hi);
}

static inline void
kfpu_restore_fxsr(struct fxregs_state *addr)
{
if (IS_ENABLED(CONFIG_X86_32)) {
kfpu_fxrstor(addr);
} else {
kfpu_fxrstorq(addr);
}
}

static inline void
kfpu_restore_fsave(struct fregs_state *addr)
{
kfpu_frstor(addr);
}

static inline void
kfpu_end(void)
{
#if defined(HAVE_KERNEL_TIF_NEED_FPU_LOAD)
union fpregs_state *state = &current->thread.fpu.state;
int error;

if (use_xsave()) {
error = copy_kernel_to_xregs_err(&state->xsave, -1);
} else if (use_fxsr()) {
error = copy_kernel_to_fxregs_err(&state->fxsave);
if (static_cpu_has(X86_FEATURE_XSAVE)) {
kfpu_restore_xsave(&current->thread.fpu.state.xsave, ~0);
} else if (static_cpu_has(X86_FEATURE_FXSR)) {
kfpu_restore_fxsr(&current->thread.fpu.state.fxsave);
} else {
error = copy_kernel_to_fregs_err(&state->fsave);
kfpu_restore_fsave(&current->thread.fpu.state.fsave);
}
WARN_ON_ONCE(error);
#endif

local_irq_enable();
preempt_enable();
}
#endif /* defined(HAVE_KERNEL_FPU) */

#else

/*
* FPU support is unavailable.
*/
#define kfpu_allowed() 0
#define kfpu_initialize(tsk) do {} while (0)
#define kfpu_begin() do {} while (0)
#define kfpu_end() do {} while (0)

#endif /* defined(HAVE_KERNEL_FPU_INTERNAL) */
#endif /* defined(KERNEL_EXPORTS_X86_FPU) */

/*
* Linux kernel provides an interface for CPU feature testing.
Expand Down

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