Re: [kvm-unit-tests RFC PATCH 09/19] arm/arm64: Zero secondary CPUs' stack

[Nikos Nikoleris <nikos.nikoleris@xxxxxxx>]

On 10/08/2022 10:42, Alexandru Elisei wrote:
> Hi Nikos,
>
> On Tue, Aug 09, 2022 at 01:56:13PM +0100, Nikos Nikoleris wrote:
> > On 09/08/2022 10:15, Alexandru Elisei wrote:
> > > For the boot CPU, the entire stack is zeroed in the entry code. For the
> > > secondaries, only struct thread_info, which lives at the bottom of the
> > > stack, is zeroed in thread_info_init().
> >
> > That's a good point.
> >
> > > Be consistent and zero the entire stack for the secondaries. This should
> > > also improve reproducibility of the testsuite, as all the stacks now start
> > > with the same contents, which is zero. And now that all the stacks are
> > > zeroed in the entry code, there is no need to explicitely zero struct
> > > thread_info in thread_info_init().
> >
> > Wouldn't it make more sense to call memset(sp, 0, THREAD_SIZE); from
> > thread_stack_alloc() instead and avoid doing this in assembly? Do we expect
>
> I prefer to do the zero'ing in assembly because:
>
> 1. For consistency, which is one of the main reasons this patch exists.
>
> 2. I don't want to deal with all the cache maintenance that is required for
> inter-CPU communication. Let's keep it simple.

I see that's a very good point. For this reason, I agree initializing to 
0 is better done locally.

Since you brought this up, we might have to worry about the thread_info 
fields we initialize in __thread_info_init(). But this is a problem for 
another patch.

Reviewed-by: Nikos Nikoleris <nikos.nikoleris@xxxxxxx>

Thanks,

Nikos

> > anyone to jump to secondary_entry without calling thread_stack_alloc()
> > first?
>
> It's impossible to jump to secondary_data.entry without allocating the
> stack first, because it's impossible to run C code without a valid stack.
>  > Thanks,
> Alex
>
> > Thanks,
> >
> > Nikos
> >
> > > Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> > > ---
> > >    arm/cstart.S          | 6 ++++++
> > >    arm/cstart64.S        | 3 +++
> > >    lib/arm/processor.c   | 1 -
> > >    lib/arm64/processor.c | 1 -
> > >    4 files changed, 9 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arm/cstart.S b/arm/cstart.S
> > > index 39260e0fa470..39e70f40986a 100644
> > > --- a/arm/cstart.S
> > > +++ b/arm/cstart.S
> > > @@ -151,7 +151,13 @@ secondary_entry:
> > >    	 */
> > >    	ldr	r1, =secondary_data
> > >    	ldr	r0, [r1]
> > > +	mov	r2, r0
> > > +	lsr	r2, #THREAD_SHIFT
> > > +	lsl	r2, #THREAD_SHIFT
> > > +	add	r3, r2, #THREAD_SIZE
> > > +	zero_range r2, r3, r4, r5
> > >    	mov	sp, r0
> > > +
> > >    	bl	exceptions_init
> > >    	bl	enable_vfp
> > > diff --git a/arm/cstart64.S b/arm/cstart64.S
> > > index d62360cf3859..54773676d1d5 100644
> > > --- a/arm/cstart64.S
> > > +++ b/arm/cstart64.S
> > > @@ -156,6 +156,9 @@ secondary_entry:
> > >    	/* set the stack */
> > >    	adrp	x0, secondary_data
> > >    	ldr	x0, [x0, :lo12:secondary_data]
> > > +	and	x1, x0, #THREAD_MASK
> > > +	add	x2, x1, #THREAD_SIZE
> > > +	zero_range x1, x2
> > >    	mov	sp, x0
> > >    	/* finish init in C code */
> > > diff --git a/lib/arm/processor.c b/lib/arm/processor.c
> > > index 9d5759686b73..ceff1c0a1bd2 100644
> > > --- a/lib/arm/processor.c
> > > +++ b/lib/arm/processor.c
> > > @@ -117,7 +117,6 @@ void do_handle_exception(enum vector v, struct pt_regs *regs)
> > >    void thread_info_init(struct thread_info *ti, unsigned int flags)
> > >    {
> > > -	memset(ti, 0, sizeof(struct thread_info));
> > >    	ti->cpu = mpidr_to_cpu(get_mpidr());
> > >    	ti->flags = flags;
> > >    }
> > > diff --git a/lib/arm64/processor.c b/lib/arm64/processor.c
> > > index 831207c16587..268b2858f0be 100644
> > > --- a/lib/arm64/processor.c
> > > +++ b/lib/arm64/processor.c
> > > @@ -232,7 +232,6 @@ void install_vector_handler(enum vector v, vector_fn fn)
> > >    static void __thread_info_init(struct thread_info *ti, unsigned int flags)
> > >    {
> > > -	memset(ti, 0, sizeof(struct thread_info));
> > >    	ti->cpu = mpidr_to_cpu(get_mpidr());
> > >    	ti->flags = flags;
> > >    }