Re: [PATCH 17/22] x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD

Borislav Petkov
2019-01-30T12:54:02Z

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.

Good mailing practices for 400: avoid top-posting and trim the reply.