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SubjectRe: [PATCH 11/11] x86/fpu: defer FPU state load until return to userspace
On Thu, Oct 4, 2018 at 7:06 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> From: Rik van Riel <riel@surriel.com>
>
> Defer loading of FPU state until return to userspace. This gives
> the kernel the potential to skip loading FPU state for tasks that
> stay in kernel mode, or for tasks that end up with repeated
> invocations of kernel_fpu_begin.
>
> It also increases the chances that a task's FPU state will remain
> valid in the FPU registers until it is scheduled back in, allowing
> us to skip restoring that task's FPU state altogether.
>
> The __fpregs_changes_{begin|end}() section ensures that the register
> remain unchanged. Otherwise a context switch or a BH could save the
> registers to its FPU context and processor's FPU register would remain
> random.
> fpu__restore() has one user so I pulled that preempt_disable() part into
> fpu__restore(). While the function did *load* the registers, it now just
> makes sure that they are loaded on return to userland.
>
> KVM swaps the host/guest register on enry/exit path. I kept the flow as
> is. First it ensures that the registers are loaded and then saves the
> current (host) state before it loads the guest's register. Before
> entring the guest, it ensures that the register are still loaded.
>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> arch/x86/entry/common.c | 9 +++
> arch/x86/include/asm/fpu/api.h | 11 +++
> arch/x86/include/asm/fpu/internal.h | 25 ++++---
> arch/x86/include/asm/trace/fpu.h | 5 +-
> arch/x86/kernel/fpu/core.c | 108 ++++++++++++++++++++--------
> arch/x86/kernel/fpu/signal.c | 3 -
> arch/x86/kernel/process.c | 2 +-
> arch/x86/kernel/process_32.c | 7 +-
> arch/x86/kernel/process_64.c | 7 +-
> arch/x86/kvm/x86.c | 18 +++--
> 10 files changed, 143 insertions(+), 52 deletions(-)
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 3b2490b819181..3dad5c3b335eb 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -31,6 +31,7 @@
> #include <asm/vdso.h>
> #include <linux/uaccess.h>
> #include <asm/cpufeature.h>
> +#include <asm/fpu/api.h>
>
> #define CREATE_TRACE_POINTS
> #include <trace/events/syscalls.h>
> @@ -196,6 +197,14 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
> if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
> exit_to_usermode_loop(regs, cached_flags);
>
> + /* Reload ti->flags; we may have rescheduled above. */
> + cached_flags = READ_ONCE(ti->flags);
> +
> + if (unlikely(cached_flags & _TIF_LOAD_FPU))
> + switch_fpu_return();
> + else
> + fpregs_is_state_consistent();

Shouldn't this be:

fpregs_assert_state_consistent(); /* see below */

if (unlikely(cached_flags & _TIF_LOAD_FPU))
switch_fpu_return();

> diff --git a/arch/x86/include/asm/fpu/api.h b/arch/x86/include/asm/fpu/api.h
> index a9caac9d4a729..e3077860f7333 100644
> --- a/arch/x86/include/asm/fpu/api.h
> +++ b/arch/x86/include/asm/fpu/api.h
> @@ -27,6 +27,17 @@ extern void kernel_fpu_begin(void);
> extern void kernel_fpu_end(void);
> extern bool irq_fpu_usable(void);
>
> +#ifdef CONFIG_X86_DEBUG_FPU
> +extern void fpregs_is_state_consistent(void);
> +#else
> +static inline void fpregs_is_state_consistent(void) { }
> +#endif

Can you name this something like fpregs_assert_state_consistent()?
The "is" name makes it sound like it's:

bool fpregs_is_state_consistent();

and you're supposed to do:

WARN_ON(!fpregs_is_state_consistent());

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