On Tue, Jun 1, 2021, at 11:06 AM, Dave Hansen wrote:
> On to patch 3:
>
> > fpu__clear_all() and fpu__clear_user_states() share an implementation,
> > and the resulting code is almost unreadable. Clean it up.
>
> Could we flesh this changelog out a bit? I basically write these in the
> process of understanding what the patch does, so this is less of "your
> changelog needs help" and more of, "here's some more detail if you want it":
>
> fpu__clear() currently resets both register state and kernel XSAVE
> buffer state. It has two modes: one for all state (supervisor and user)
> and another for user state only. fpu__clear_all() uses the "all state"
> (user_only=0) mode, while a number of signal paths use the user_only=1 mode.
>
> Make fpu__clear() work only for user state (user_only=1) and remove the
> "all state" (user_only=0) code. Rename it to match so it can be used by
> the signal paths.
>
> Replace the "all state" (user_only=0) fpu__clear() functionality. Use
> the TIF_NEED_FPU_LOAD functionality instead of making any actual
> hardware registers changes in this path.
I’ll steal some of this.
>
> > arch/x86/kernel/fpu/core.c | 63 +++++++++++++++++++++++++-------------
> > 1 file changed, 42 insertions(+), 21 deletions(-)
> >
> > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> > index 571220ac8bea..a01cbb6a08bb 100644
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -354,45 +354,66 @@ static inline void copy_init_fpstate_to_fpregs(u64 features_mask)
> > }
> >
> > /*
> > - * Clear the FPU state back to init state.
> > - *
> > - * Called by sys_execve(), by the signal handler code and by various
> > - * error paths.
> > + * Reset current's user FPU states to the init states. current's supervisor
> > + * states, if any, are not modified by this function. The XSTATE header
> > + * in memory is assumed to be intact when this is called.
> > */
> > -static void fpu__clear(struct fpu *fpu, bool user_only)
> > +void fpu__clear_user_states(struct fpu *fpu)
> > {
> > WARN_ON_FPU(fpu != ¤t->thread.fpu);
> >
> > if (!static_cpu_has(X86_FEATURE_FPU)) {
> > - fpu__drop(fpu);
> > - fpu__initialize(fpu);
> > + fpu__clear_all(fpu);
> > return;
> > }
> >
> > fpregs_lock();
> >
> > - if (user_only) {
> > - if (!fpregs_state_valid(fpu, smp_processor_id()) &&
> > - xfeatures_mask_supervisor())
> > - copy_kernel_to_xregs(&fpu->state.xsave,
> > - xfeatures_mask_supervisor());
> > - copy_init_fpstate_to_fpregs(xfeatures_mask_user());
> > - } else {
> > - copy_init_fpstate_to_fpregs(xfeatures_mask_all);
> > - }
> > + /*
> > + * Ensure that current's supervisor states are loaded into
> > + * their corresponding registers.
> > + */
> > + if (!fpregs_state_valid(fpu, smp_processor_id()) &&
> > + xfeatures_mask_supervisor())
> > + copy_kernel_to_xregs(&fpu->state.xsave,
> > + xfeatures_mask_supervisor());
> >
> > + /*
> > + * Reset user states in registers.
> > + */
> > + copy_init_fpstate_to_fpregs(xfeatures_mask_user());
> > +
> > + /*
> > + * Now all FPU registers have their desired values. Inform the
> > + * FPU state machine that current's FPU registers are in the
> > + * hardware registers.
> > + */
> > fpregs_mark_activate();
> > +
> > fpregs_unlock();
> > }
> >
> > -void fpu__clear_user_states(struct fpu *fpu)
> > -{
> > - fpu__clear(fpu, true);
> > -}
> > +/*
> > + * Reset current's FPU registers (user and supervisor) to their INIT values.
> > + * This is used by execve(); out of an abundance of caution, it completely
> > + * wipes and resets the XSTATE buffer in memory.
> > + *
> > + * Note that XSAVE (unlike XSAVES) expects the XSTATE buffer in memory to
> > + * be valid, so there are certain forms of corruption of the XSTATE buffer
> > + * in memory that would survive initializing the FPU registers and XSAVEing
> > + * them to memory.
> > + *
> > + * This does not change the actual hardware registers; when fpu__clear_all()
> > + * returns, TIF_NEED_FPU_LOAD will be set, and a subsequent exit to user mode
> > + * will reload the hardware registers from memory.
> > + */
> > void fpu__clear_all(struct fpu *fpu)
> > {
> > - fpu__clear(fpu, false);
> > + fpregs_lock();
> > + fpu__drop(fpu);
> > + fpu__initialize(fpu);
> > + fpregs_unlock();
> > }
>
> Nit: Could we move the detailed comments about TIF_NEED_FPU_LOAD right
> next to the fpu__initialize() call? It would make it painfully obvious
> which call is responsible. The naming isn't super helpful here.
>
Hmm. I was actually thinking of open-coding it all. fpu__initialize() and fpu__drop() have very few callers, and I’m not even convinced that the other callers are calling them for a valid reason.
