[PATCH -next v13 10/19] riscv: Allocate user's vector context in the first-use trap

[Vineet Gupta]

On 2/7/23 06:36, Björn Töpel wrote:
>> +bool rvv_first_use_handler(struct pt_regs *regs)
>> +{
>> +	__user u32 *epc = (u32 *)regs->epc;
>> +	u32 tval = (u32)regs->badaddr;
>> +
>> +	/* If V has been enabled then it is not the first-use trap */
>> +	if (vstate_query(regs))
>> +		return false;
>> +	/* Get the instruction */
>> +	if (!tval) {
>> +		if (__get_user(tval, epc))
>> +			return false;
>> +	}
>> +	/* Filter out non-V instructions */
>> +	if (!insn_is_vector(tval))
>> +		return false;
>> +	/* Sanity check. datap should be null by the time of the first-use trap */
>> +	WARN_ON(current->thread.vstate.datap);
>> +	/*
>> +	 * Now we sure that this is a V instruction. And it executes in the
>> +	 * context where VS has been off. So, try to allocate the user's V
>> +	 * context and resume execution.
>> +	 */
>> +	if (rvv_thread_zalloc()) {
>> +		force_sig(SIGKILL);
>> +		return true;
>> +	}
> Should the altstack size be taken into consideration, like x86 does in
> validate_sigaltstack() (see __xstate_request_perm()).

For a preexisting alternate stack ? Otherwise there is no 
"configuration" like x86 to cross-check against and V fault implies 
large'ish signal stack.
See below as well.

> Related; Would it make sense to implement sigaltstack_size_valid() for
> riscv, analogous to x86?

Indeed we need to do that for the case where alt stack is being setup, 
*after* V fault-on-first use.
But how to handle an existing alt stack which might not be big enough to 
handle V state ?

-Vineet