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

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Message-ID: <20190207141027.ig3vjozwndspuwiw@linutronix.de>
Date:   Thu, 7 Feb 2019 15:10:27 +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 13:53:51 [+0100], Borislav Petkov wrote:
> > 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);

The (your) above example yes. But the reverse state
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
 		do_that()

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

> without the "!". I.e., CPU's FPU regs are valid and we need to save them.
> 
> Or am I misreading the comment above?

Your example is correct. But in the opposite case, when ! was not there
then we have to add it.

> > 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?

if you need to LOAD then task-saved-area into CPU-state. If you need to
save it then CPU-state into task-saved-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.

hmmm. I think it is just taste / habit.

> > 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.

If it would be just a name thing then I probably wouldn't mind. But
swapping the logic might break things so I try to avoid that.

> Thx.
> 

Sebastian