On Wed, May 23, 2018 at 09:15:11PM +0100, Alex Bennée wrote:
>
> Dave Martin <Dave.Martin@xxxxxxx> writes:
>
> > In preparation for allowing non-task (i.e., KVM vcpu) FPSIMD
> > contexts to be handled by the fpsimd common code, this patch adapts
> > task_fpsimd_save() to save back the currently loaded context,
> > removing the explicit dependency on current.
> >
> > The relevant storage to write back to in memory is now found by
> > examining the fpsimd_last_state percpu struct.
> >
> > fpsimd_save() does nothing unless TIF_FOREIGN_FPSTATE is clear, and
> > fpsimd_last_state is updated under local_bh_disable() or
> > local_irq_disable() everywhere that TIF_FOREIGN_FPSTATE is cleared:
> > thus, fpsimd_save() will write back to the correct storage for the
> > loaded context.
> >
> > No functional change.
> >
> > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> > Acked-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> > Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> > ---
> > arch/arm64/kernel/fpsimd.c | 25 +++++++++++++------------
> > 1 file changed, 13 insertions(+), 12 deletions(-)
> >
> > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > index 9d85373..3aa100a 100644
> > --- a/arch/arm64/kernel/fpsimd.c
> > +++ b/arch/arm64/kernel/fpsimd.c
> > @@ -270,13 +270,15 @@ static void task_fpsimd_load(void)
> > }
> >
> > /*
> > - * Ensure current's FPSIMD/SVE storage in thread_struct is up to date
> > - * with respect to the CPU registers.
> > + * Ensure FPSIMD/SVE storage in memory for the loaded context is up to
> > + * date with respect to the CPU registers.
> > *
> > * Softirqs (and preemption) must be disabled.
> > */
> > -static void task_fpsimd_save(void)
> > +static void fpsimd_save(void)
> > {
> > + struct user_fpsimd_state *st = __this_cpu_read(fpsimd_last_state.st);
> > +
>
> I thought I was missing something but the only write I saw of this was:
>
> __this_cpu_write(fpsimd_last_state.st, NULL);
>
> which implied to me it is possible to have an invalid de-reference. I
> did figure it out eventually as fpsimd_bind_state_to_cpu uses a more
> indirect this_cpu_ptr idiom for tweaking this. I guess a reference to
> fpsimd_bind_[task|state]_to_cpu in the comment would have helped my
> confusion.
How about:
static void fpsimd_save(void)
{
struct user_fpsimd_state *st = __this_cpu_read(fpsimd_last_state.st);
+ /* set by fpsimd_bind_to_cpu() */
WARN_ON(!in_softirq() && !irqs_disabled());
> Reviewed-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
Thanks
---Dave
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