Dave Martin <Dave.Martin@xxxxxxx> writes:
> 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() */
Great, thanks ;-)
--
Alex Bennée
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