linux-kernel - Re: [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU

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Date:   Wed, 30 Jan 2019 13:28:20 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Borislav Petkov <bp@...en8.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 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for
 TIF_NEED_FPU_LOAD

On 2019-01-30 12:56:14 [+0100], Borislav Petkov wrote:
> > diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> > index bf4e6caad305e..a25be217f9a2c 100644
> > --- a/arch/x86/kernel/fpu/signal.c
> > +++ b/arch/x86/kernel/fpu/signal.c
> > @@ -156,7 +156,16 @@ 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;
> >  
> > -	copy_fpregs_to_fpstate(fpu);
> > +	__fpregs_changes_begin();
> > +	/*
> > +	 * If we do not need to load the FPU registers at return to userspace
> > +	 * then the CPU has the current state and we need to save it. Otherwise
> > +	 * it is already done and we can skip it.
> > +	 */
> > +	if (!test_thread_flag(TIF_NEED_FPU_LOAD))
> > +		copy_fpregs_to_fpstate(fpu);
> 
> I wonder if this flag would make the code more easy to follow by calling
> it
> 
> 	TIF_FPU_REGS_VALID
> 
> instead, to denote that the FPU registers in the CPU have a valid
> content.
> 
> Then the test becomes:
> 
> 	if (test_thread_flag(TIF_FPU_REGS_VALID))
> 		copy_fpregs_to_fpstate(fpu);

I've been asked to add comment above the sequence so it is understood. I
think the general approach is easy to follow once the concept is
understood. I don't mind renaming the TIF_ thingy once again (it
happend once or twice and I think the current one was suggested by Andy
unless I mixed things up).
The problem I have with the above is that

	if (test_thread_flag(TIF_NEED_FPU_LOAD))
		do_that()

becomes
	if (!test_thread_flag(TIF_FPU_REGS_VALID))
		do_that()

and you could argue again the other way around. So do we want NEED_LOAD
or NEED_SAVE flag which is another way of saying REGS_VALID?
More importantly the logic is changed when the bit is set and this
requires more thinking than just doing sed on the patch series.

Sebastian