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

Sebastian Andrzej Siewior
2019-01-30T12:28:27Z

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