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 13:53:51 +0100
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 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for
 TIF_NEED_FPU_LOAD

On Wed, Jan 30, 2019 at 01:28:20PM +0100, Sebastian Andrzej Siewior wrote:
> > > +	/*
> > > +	 * 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()

Err, above it becomes

	if (test_thread_flag(TIF_FPU_REGS_VALID))
		copy_fpregs_to_fpstate(fpu);

without the "!". I.e., CPU's FPU regs are valid and we need to save them.

Or am I misreading the comment above?

> 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?

All fine and dandy except NEED_FPU_LOAD is ambiguous to me: we need to
load them where? Into the CPU? Or into the FPU state save area?

TIF_FPU_REGS_VALID is clearer in the sense that the CPU's FPU registers
are currently valid for the current task. As there are no other FPU
registers except the CPU's.

> More importantly the logic is changed when the bit is set and this
> requires more thinking than just doing sed on the patch series.

Sure.

And I'll get accustomed to the logic whatever the name is - this is just
a "wouldn't it be better" thing.

Thx.

-- 
Regards/Gruss,
    Boris.

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