linux-kernel - Re: [PATCH 08/11] x86/fpu: Always store the registers in copy_fpstate_to

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Date:   Fri, 2 Nov 2018 15:42:28 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
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>
Subject: Re: [PATCH 08/11] x86/fpu: Always store the registers in
 copy_fpstate_to_sigframe()

On 2018-10-12 11:15:51 [-0700], Dave Hansen wrote:
> > @@ -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?

Since the ->initialized is gone, the whole hunk here looks differently
and probably easier to understand.

> > +	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;
> >  	}

dropped this.

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

Sebastian