Re: [PATCH v2] riscv: add irq stack support

[Arnd Bergmann <arnd@arndb.de>]

On Mon, May 23, 2022 at 10:16 AM Arnd Bergmann <arnd@arndb.de> wrote:
> > > > +       for_each_possible_cpu(cpu) {
> > > > +#ifdef CONFIG_VMAP_STACK
> > > > +               void *s = __vmalloc_node(IRQ_STACK_SIZE, THREAD_ALIGN,
> > > > +                                        THREADINFO_GFP, cpu_to_node(cpu),
> > > > +                                        __builtin_return_address(0));
> > > > +#else
> > > > +               void *s = (void *)__get_free_pages(GFP_KERNEL, get_order(IRQ_STACK_SIZE));
> > > > +#endif
> > >
> > > On a related topic: is there a reason to still keep the non-VMAP_STACK
> >
> > irq stack is 16KB on RV64 now, vmalloc doesn't gurantee physical
> > continuous pages, I want to keep the stack physical continuous
> > characteristic for !VMAP_STACK case.
>
> I don't understand. What is the benefit of having a physically continuous
> stack? If this is required for something, you could still get that with a VMAP
> stack by using alloc_pages() to allocate the stack and them using vmap() to
> put it into the vmalloc range with appropriate guard pages.
>
> I think we really want to avoid the case of missing guard pages around
> the stack, and eliminate the part where the stack is in the linear map.

I was actually confused here and mixed up a few things: I thought this
was about whether to use vmap stacks unconditionally, and this is in
fact not even an architecture specific decision, it's a global option as you
are probably aware.

Since one can still turn off VMAP_STACK for normal thread stacks,
it doesn't make much of a difference whether one can do the same for
IRQ stacks. Please just ignore what I said above. I see you already
sent a modified v3, and I think either way is fine, feel free to revert back
to this method if it makes life easier.

       Arnd