Re: [PATCH v3 2/4] riscv: add support for SBI Supervisor Software Events extension

["Clément Léger" <cleger@rivosinc.com>]

On 27/01/2025 09:09, Alexandre Ghiti wrote:
>> I believe the goal is not the same. Using CONFIG_VMAP_STACK allows the
>> kernel exception handling to catch any stack overflow when entering the
>> kernel and thus using vmalloc is required to allocate twice the page
>> size (overflow is when sp is located in the upper half of the allocated
>> vmalloc stack. So basically, this is two distinct purposes.
>>
>> AFAIU, kvmalloc allows to fallback to vmalloc if kmalloc fails. This is
>> not what we are looking for here since our allocation size is always
>> quite small and known (STACK_SIZE basically).
>>
>> But I might be missing something.
> 
> 
> arch_alloc_vmap_stack() only vmalloc the stack and does not implement
> any stack overflow mechanism, so I'm still unsure we need the define.

Hi Alex,

So actually, the stack overflow check itself is done in the exception
entry. It check if the stack pointer did passed in the upper part of the
vmalloc allocation (see entry.S:122). In this allocation, the stack size
is actually * 2:

#ifdef CONFIG_VMAP_STACK
#define THREAD_ALIGN            (2 * THREAD_SIZE)
#else
#define THREAD_ALIGN            THREAD_SIZE
#endif

So even though it does nothing special by itself, it centralize the
allocation size/method. And size the size is larger, using vamlloc makes
sense I guess. The same mechanism is used to allocate irq stack as well.

Thanks,

Clément

> 
> Thanks,
> 
> Alex