Re: [PATCH] random: align entropy_timer_state to cache line

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Nov 30, 2022 at 11:04:23AM +0100, Jason A. Donenfeld wrote:
> > > diff --git a/drivers/char/random.c b/drivers/char/random.c
> > > index 67558b95d531..2494e08c76d8 100644
> > > --- a/drivers/char/random.c
> > > +++ b/drivers/char/random.c
> > > @@ -1262,7 +1262,7 @@ static void __cold entropy_timer(struct timer_list *timer)
> > >  static void __cold try_to_generate_entropy(void)
> > >  {
> > >  	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 15 };
> > > -	struct entropy_timer_state stack;
> > > +	struct entropy_timer_state stack ____cacheline_aligned;
> > 
> > Several years ago, there was a whole thing about how __attribute__((aligned)) to
> > more than 8 bytes doesn't actually work on stack variables in the kernel on x86,
> > because the kernel only keeps the stack 8-byte aligned but gcc assumes it is
> > 16-byte aligned.  See
> > https://lore.kernel.org/linux-crypto/20170110143340.GA3787@gondor.apana.org.au/T/#t
> > 
> > IIRC, nothing was done about it at the time.
> > 
> > Has that been resolved in the intervening years?
> 
> Maybe things are different for ____cacheline_aligned, which is 64 bytes.
> Reading that thread, it looks like it was a case of trying to align the
> stack to 16 bytes, but gcc assumed 16 bytes already while the kernel
> only gave it 8. So gcc didn't think it needed to emit any code to align
> it. Here, though, it's 64, and gcc certainly isn't assuming 64-byte
> stack alignment.
> 
> Looking at the codegen, gcc appears to doing `rsp = (rsp & ~63) - 64`,
> which appears correct.

Well, if gcc thinks the stack is already 16-byte aligned, then it would be
perfectly within its rights to do 'rsp = (rsp & ~47) - 64', right?  You probably
don't want to be relying on an implementation detail of gcc codegen...

- Eric