linux-kernel - Re: [PATCH v2] riscv: add irq stack support

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Date:   Mon, 23 May 2022 10:16:56 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] riscv: add irq stack support

On Sun, May 15, 2022 at 7:14 AM Jisheng Zhang <jszhang@...nel.org> wrote:
> On Mon, Mar 07, 2022 at 08:19:35PM +0100, Arnd Bergmann wrote:
> > On Mon, Mar 7, 2022 at 3:08 PM Jisheng Zhang <jszhang@...nel.org> wrote:
> > > +2:
> >
> > What is the benefit of doing this in assembler? Is it measurably faster?
> >
> > I see that arm64 does the same thing in C code, and it would be best to
> > have a common implementation for doing this, in terms of maintainability.
> >
>
> Sorry for delay. The assembler code is mainly to cal the stack ptr then
> change the SP to use the stack, which equals to arm64 call_on_irq_stack()
> which is implemented in assembler too.

I understand that you need to be in asm code to switch the stack, it
just felt that the arm64 method is a bit easier to debug here.

I suppose being able to keep using generic_handle_arch_irq() is also
beneficial, so it doesn't make much difference either way.

> > > +       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.

        Arnd