[PATCH v2] riscv: add irq stack support

[David Laight]

From: Jisheng Zhang
> Sent: 07 March 2022 14:08
> Currently, IRQs are still handled on the kernel stack of the current
> task on riscv platforms. If the task has a deep call stack at the time
> of interrupt, and handling the interrupt also requires a deep stack,
> it's possible to see stack overflow.
> 
...
I'd have thought that a single page is (probably) enough for the
IRQ stack.
Certainly its sizing isn't really related to the normal thread
stack size.

> From another side, after this patch, it's possible to reduce the
> THREAD_SIZE to 8KB for RV64 platforms. This is especially useful for
> those systems with small memory size, e.g the Allwinner D1S platform
> which is RV64 but only has 64MB DDR.

Are you sure?
Is the stack use likely to be very much less than that of x86-64?
The real problem isn't the stack use of the test you are doing,
but the horrid worst case stack of some path that has multiple
1k+ buffers on stack.

Apart from compiler fubar (which usually hit KASAN) that stack
is actually likely to be architecture independent.
(The difference between 32bit and 64bit is also likely to be
relatively small - unless there are on-stack arrays of 'long'.)

For VMAP stacks is there a 'guard' KVA page allocated below
all of the stacks?
64bit systems should have lots of KVA so this shouldn't be
a problem.
Then stack overruns will fault and panic rather than trashing
another data area - which is really hard to debug.

	David

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