Re: [PATCH 24/27] x86/fpu: Add a fastpath to __fpu__restore_sig()

Borislav Petkov
2019-04-12T17:18:10Z

On Wed, Apr 03, 2019 at 06:41:53PM +0200, 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) {
> 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 {

I've added here:

+ /*
+ * Attempt to restore the FPU registers directly from user
+ * memory. For that to succeed, the user accesses cannot cause
+ * page faults. If they do, fall back to the slow path below,
+ * going through the kernel buffer.
+ */

so that it is clear what's happening.

This function is doing gazillion things again ;-\

--
Regards/Gruss,
Boris.

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