Re: [RFC PATCH] arm64/fpsimd: Don't disable softirq when touching FPSIMD/SVE state

[Julien Grall]

Hi Dave,

On 4/5/19 4:07 PM, Dave Martin wrote:
> On Fri, Apr 05, 2019 at 10:02:45AM +0100, Julien Grall wrote:
> > > > +#ifdef CONFIG_KERNEL_MODE_NEON
> > > > +
> > > >   /*
> > > >    * may_use_simd - whether it is allowable at this time to issue SIMD
> > > >    *                instructions or access the SIMD register file
> > > > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > > > index 5ebe73b69961..b7e5dac26190 100644
> > > > --- a/arch/arm64/kernel/fpsimd.c
> > > > +++ b/arch/arm64/kernel/fpsimd.c
> > > > @@ -90,7 +90,8 @@
> > > >    * To prevent this from racing with the manipulation of the task's FPSIMD state
> > > >    * from task context and thereby corrupting the state, it is necessary to
> > > >    * protect any manipulation of a task's fpsimd_state or TIF_FOREIGN_FPSTATE
> > > > - * flag with local_bh_disable() unless softirqs are already masked.
> > > > + * flag with kernel_neon_{disable, enable}. This will still allow softirqs to
> > > > + * run but prevent them to use FPSIMD.
> > > >    *
> > > >    * For a certain task, the sequence may look something like this:
> > > >    * - the task gets scheduled in; if both the task's fpsimd_cpu field
> > > > @@ -142,6 +143,9 @@ extern void __percpu *efi_sve_state;
> > > >   #endif /* ! CONFIG_ARM64_SVE */
> > > > +static void kernel_neon_disable(void);
> > > > +static void kernel_neon_enable(void);
> > >
> > > I find these names a bit confusing: _disable() actualy enables FPSIMD/SVE
> > > context access for the current context (i.e., makes it safe).
> > >
> > > Since these also disable/enable preemption, perhaps we can align them
> > > with the existing get_cpu()/put_cpu(), something like:
> > >
> > > void get_cpu_fpsimd_context();
> > > vold put_cpu_fpsimd_context();
> > >
> > > If you consider it's worth adding the checking helper I alluded to
> > > above, it could then be called something like:
> > >
> > > bool have_cpu_fpsimd_context();
> >
> > I am not sure where you suggested a checking helper above. Do you refer to
> > exposing kernel_neon_busy even for !CONFIG_KERNEL_MODE_NEON?
>
> Hmmm, looks like I got my reply out of order here.
>
> I meant the helper (if any) to check
> !preemptible() && !__this_cpu_read(kernel_neon_busy).

I guess you are using && instead of || because some of the caller may 
not call get_cpu_fpsimd_context() before but still disable preemption, 
right?

Wouldn't it be better to have all the user calling 
get_cpu_fpsimd_context() and put_cpu_fpsimd_context()?

This has the advantage to uniformize how the way FPSIMD is protected and 
also...

> Looks like you inferred what I meant later on anyway.
>
> > > > +
> > > >   /*
> > > >    * Call __sve_free() directly only if you know task can't be scheduled
> > > >    * or preempted.
> > > > @@ -213,11 +217,11 @@ static void sve_free(struct task_struct *task)
> > > >    * thread_struct is known to be up to date, when preparing to enter
> > > >    * userspace.
> > > >    *
> > > > - * Softirqs (and preemption) must be disabled.
> > > > + * Preemption must be disabled.
> > >
> > > [*] That's not enough: we need to be in kernel_neon_disable()...
> > > _enable() (i.e., kernel_neon_busy needs to be true to prevent softirqs
> > > messing with the FPSIMD state).
> >
> > How about not mentioning preemption at all and just say:
> >
> > "The fpsimd context should be acquired before hand".
> >
> > This would help if we ever decide to protect critical section differently.
>
> Yes, or even better, name the function used to do this (i.e.,
> kernel_neon_disable() or get_cpu_fpsimd_context() or whatever it's
> called).

... would make the comments simpler because we would have only one 
possible case to care.

Cheers,

--
Julien Grall