linux-kernel - Re: [PATCH 24/27] x86/fpu: Add a fastpath to __fpu__restore

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Date:   Fri, 12 Apr 2019 19:17:59 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 24/27] x86/fpu: Add a fastpath to __fpu__restore_sig()

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@...utronix.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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