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

[Alexandru Elisei <alexandru.elisei@arm.com>]

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.

> 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@arm.com>
> > ---
> >   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;
> >   }
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