Re: [PATCH 08/11] x86/fpu: Always store the registers in copy_fpstate_to_sigframe()

[Christophe de Dinechin <christophe.de.dinechin@gmail.com>]

Sebastian Andrzej Siewior writes:

> 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.

One of the callers is in a different file.
Not sure if that warrants some kind of check that
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

s/relict/relic/

> 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.

register [are] always loaded

> 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.

maybe "only on return"?

>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  arch/x86/include/asm/fpu/internal.h | 45 ----------------------------
>  arch/x86/kernel/fpu/signal.c        | 46 +++++++----------------------
>  2 files changed, 11 insertions(+), 80 deletions(-)
>
> 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.
>   *

I would modify the function comment to add the new expectation that
fpu->initialized must be set on entry.

> @@ -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);
> +	}
>
> +	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;
>  	}


--
Cheers,
Christophe de Dinechin (IRC c3d)