From mboxrd@z Thu Jan  1 00:00:00 1970
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for
 TIF_NEED_FPU_LOAD
Date: Thu, 7 Feb 2019 15:10:27 +0100
Message-ID: <20190207141027.ig3vjozwndspuwiw@linutronix.de>
References: <20190109114744.10936-1-bigeasy@linutronix.de>
 <20190109114744.10936-18-bigeasy@linutronix.de>
 <20190130115614.GF18383@zn.tnic>
 <20190130122820.qyyn4vbtkqelmp54@linutronix.de>
 <20190130125351.GH18383@zn.tnic>
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Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        kvm@vger.kernel.org, "Jason A. Donenfeld" <Jason@zx2c4.com>,
        Rik van Riel <riel@surriel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>
To: Borislav Petkov <bp@alien8.de>
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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