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Subject: Re: [PATCH 08/11] x86/fpu: Always store the registers in
 copy_fpstate_to_sigframe()
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        linux-kernel@vger.kernel.org
References: <20181004140547.13014-1-bigeasy@linutronix.de>
 <20181004140547.13014-9-bigeasy@linutronix.de>
Cc:     x86@kernel.org, Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Rik van Riel <riel@surriel.com>
From:   Dave Hansen <dave.hansen@linux.intel.com>
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Message-ID: <53f013ca-d6ff-2387-f9b0-d6c6df66d082@linux.intel.com>
Date:   Fri, 12 Oct 2018 11:15:51 -0700
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On 10/04/2018 07:05 AM, Sebastian Andrzej Siewior wrote:
> From: Rik van Riel <riel@surriel.com>
> 
> copy_fpstate_to_sigframe() has two callers and both invoke the function only if
> fpu->initialized is set. So the check in the function for ->initialized makes
> no sense. It might be a relict from the lazy-FPU time: If the FPU registers
> were "loaded" then we could save them directly. Otherwise (the FPU
> registers are not up to date) they are saved in the FPU struct and
> it could be used for memcpy().
> 
> Since the registers always loaded at this point, save them and copy
> later.
> This code is extracted from an earlier version of the patchset while
> there still was lazy-FPU on x86. This is a preparation for loading the
> FPU registers on return to userland.

Could you make a pass through all the changelogs at at least make sure
they look sane and consistent?  The text width and paragraph spacing
here looks rather cobbled together.

> ---
>  arch/x86/include/asm/fpu/internal.h | 45 ----------------------------
>  arch/x86/kernel/fpu/signal.c        | 46 +++++++----------------------
>  2 files changed, 11 insertions(+), 80 deletions(-)

The diffstat here is really nice.

> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
> index 4ecaf4d22954e..df8816be3efdd 100644
> --- a/arch/x86/include/asm/fpu/internal.h
> +++ b/arch/x86/include/asm/fpu/internal.h
> @@ -126,22 +126,6 @@ extern void fpstate_sanitize_xstate(struct fpu *fpu);
>  		     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_fprestore)	\
>  		     : output : input)
>  
> -static inline int copy_fregs_to_user(struct fregs_state __user *fx)
> -{
> -	return user_insn(fnsave %[fx]; fwait,  [fx] "=m" (*fx), "m" (*fx));
> -}
> -
> -static inline int copy_fxregs_to_user(struct fxregs_state __user *fx)
> -{
> -	if (IS_ENABLED(CONFIG_X86_32))
> -		return user_insn(fxsave %[fx], [fx] "=m" (*fx), "m" (*fx));
> -	else if (IS_ENABLED(CONFIG_AS_FXSAVEQ))
> -		return user_insn(fxsaveq %[fx], [fx] "=m" (*fx), "m" (*fx));
> -
> -	/* See comment in copy_fxregs_to_kernel() below. */
> -	return user_insn(rex64/fxsave (%[fx]), "=m" (*fx), [fx] "R" (fx));
> -}
> -
>  static inline void copy_kernel_to_fxregs(struct fxregs_state *fx)
>  {
>  	if (IS_ENABLED(CONFIG_X86_32)) {
> @@ -352,35 +336,6 @@ static inline void copy_kernel_to_xregs(struct xregs_state *xstate, u64 mask)
>  	XSTATE_XRESTORE(xstate, lmask, hmask);
>  }
>  
> -/*
> - * Save xstate to user space xsave area.
> - *
> - * We don't use modified optimization because xrstor/xrstors might track
> - * a different application.
> - *
> - * We don't use compacted format xsave area for
> - * backward compatibility for old applications which don't understand
> - * compacted format of xsave area.
> - */
> -static inline int copy_xregs_to_user(struct xregs_state __user *buf)
> -{
> -	int err;
> -
> -	/*
> -	 * Clear the xsave header first, so that reserved fields are
> -	 * initialized to zero.
> -	 */
> -	err = __clear_user(&buf->header, sizeof(buf->header));
> -	if (unlikely(err))
> -		return -EFAULT;
> -
> -	stac();
> -	XSTATE_OP(XSAVE, buf, -1, -1, err);
> -	clac();
> -
> -	return err;
> -}
> -
>  /*
>   * Restore xstate from user space xsave area.
>   */
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 23f1691670b66..c8f5ff58578ed 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -117,23 +117,6 @@ static inline int save_xstate_epilog(void __user *buf, int ia32_frame)
>  
>  	return err;
>  }
> -
> -static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf)
> -{
> -	int err;
> -
> -	if (use_xsave())
> -		err = copy_xregs_to_user(buf);
> -	else if (use_fxsr())
> -		err = copy_fxregs_to_user((struct fxregs_state __user *) buf);
> -	else
> -		err = copy_fregs_to_user((struct fregs_state __user *) buf);
> -
> -	if (unlikely(err) && __clear_user(buf, fpu_user_xstate_size))
> -		err = -EFAULT;
> -	return err;
> -}
> -
>  /*
>   * Save the fpu, extended register state to the user signal frame.
>   *
> @@ -172,27 +155,20 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
>  			sizeof(struct user_i387_ia32_struct), NULL,
>  			(struct _fpstate_32 __user *) buf) ? -1 : 1;
>  
> -	if (fpu->initialized || using_compacted_format()) {
> -		/* Save the live register state to the user directly. */
> -		if (copy_fpregs_to_sigframe(buf_fx))
> -			return -1;
> -		/* Update the thread's fxstate to save the fsave header. */
> -		if (ia32_fxstate)
> -			copy_fxregs_to_kernel(fpu);
> +	/* Update the thread's fxstate to save the fsave header. */
> +	if (ia32_fxstate) {
> +		copy_fxregs_to_kernel(fpu);
>  	} else {
> -		/*
> -		 * It is a *bug* if kernel uses compacted-format for xsave
> -		 * area and we copy it out directly to a signal frame. It
> -		 * should have been handled above by saving the registers
> -		 * directly.
> -		 */
> -		if (boot_cpu_has(X86_FEATURE_XSAVES)) {
> -			WARN_ONCE(1, "x86/fpu: saving compacted-format xsave area to a signal frame!\n");
> -			return -1;
> -		}
> +		copy_fpregs_to_fpstate(fpu);
> +		fpregs_deactivate(fpu);
> +	}

Could you add a high-level comment for this if{}else{} block that says
something like:

	/* Save the registers to the fpstate. */

I also think it's worthwhile to explain the asymmetry between the
ia32_fxstate case and the other branch.  Why don't we
fpregs_deactivate() in the ia32_fxstate path, for instance?

> +	if (using_compacted_format()) {
> +		copy_xstate_to_user(buf_fx, xsave, 0, size);
> +	} else {
>  		fpstate_sanitize_xstate(fpu);
> -		if (__copy_to_user(buf_fx, xsave, fpu_user_xstate_size))
> +		size = fpu_user_xstate_size;
> +		if (__copy_to_user(buf_fx, xsave, size))
>  			return -1;
>  	}

This seems unnecessary.  Why are you updating 'size' like this?