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SubjectRe: [PATCH 24/27] x86/fpu: Add a fastpath to __fpu__restore_sig()
On Wed, 3 Apr 2019, Sebastian Andrzej Siewior wrote:

> The previous commits refactor the restoration of the FPU registers so
> that they can be loaded from in-kernel memory. This overhead can be
> avoided if the load can be performed without a pagefault.
>
> Attempt to restore FPU registers by invoking
> copy_user_to_fpregs_zeroing(). If it fails try the slowpath which can handle
> pagefaults.
>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
> arch/x86/kernel/fpu/signal.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index a5b086ec426a5..f20e1d1fffa29 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -242,10 +242,10 @@ sanitize_restored_xstate(union fpregs_state *state,
> /*
> * Restore the extended state if present. Otherwise, restore the FP/SSE state.
> */
> -static inline int copy_user_to_fpregs_zeroing(void __user *buf, u64 xbv, int fx_only)
> +static int copy_user_to_fpregs_zeroing(void __user *buf, u64 xbv, int fx_only)
> {
> if (use_xsave()) {
> - if ((unsigned long)buf % 64 || fx_only) {
> + if (fx_only) {

This change is weird and not mentioned in the changelog....

> u64 init_bv = xfeatures_mask & ~XFEATURE_MASK_FPSSE;
> copy_kernel_to_xregs(&init_fpstate.xsave, init_bv);
> return copy_user_to_fxregs(buf);
> @@ -327,7 +327,19 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> if (ret)
> goto err_out;
> envp = &env;
> + } else {
> + fpregs_lock();
> + pagefault_disable();
> + ret = copy_user_to_fpregs_zeroing(buf_fx, xfeatures, fx_only);
> + pagefault_enable();
> + if (!ret) {
> + fpregs_mark_activate();
> + fpregs_unlock();
> + return 0;
> + }
> + fpregs_unlock();
> }
> +
> if (use_xsave() && !fx_only) {
> u64 init_bv = xfeatures_mask & ~xfeatures;
>
> --
> 2.20.1
>
>

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