Re: [PATCH v3 2/2] powerpc/rtas: Implement reentrant rtas call

Hello Nathan,

On Thu, 2020-05-14 at 13:58 -0500, Nathan Lynch wrote:

Hi,

Leonardo Bras [off-list ref] writes:

quoted

+/**
+ * rtas_call_reentrant() - Used for reentrant rtas calls
+ * @token:	Token for desired reentrant RTAS call
+ * @nargs:	Number of Input Parameters
+ * @nret:	Number of Output Parameters
+ * @outputs:	Array of outputs
+ * @...:	Inputs for desired RTAS call
+ *
+ * According to LoPAR documentation, only "ibm,int-on", "ibm,int-off",
+ * "ibm,get-xive" and "ibm,set-xive" are currently reentrant.
+ * Reentrant calls need their own rtas_args buffer, so not using rtas.args, but
+ * PACA one instead.
+ *
+ * Return:	-1 on error,
+ *		First output value of RTAS call if (nret > 0),
+ *		0 otherwise,
+ */
+
+int rtas_call_reentrant(int token, int nargs, int nret, int *outputs, ...)
+{
+	va_list list;
+	struct rtas_args *args;
+	int i;
+
+	if (!rtas.entry || token == RTAS_UNKNOWN_SERVICE)
+		return -1;
+
+	/* We use the per-cpu (PACA) rtas args buffer */
+	args = &local_paca->reentrant_args;
+
+	va_start(list, outputs);
+	va_rtas_call_unlocked(args, token, nargs, nret, list);
+	va_end(list);
+
+	if (nret > 1 && outputs)
+		for (i = 0; i < nret - 1; ++i)
+			outputs[i] = be32_to_cpu(args->rets[i + 1]);

Doesn't this need to be more careful about preemption and exceptions?
I.e. the args structure in the paca needs to be protected from
concurrent use somehow, like any per-cpu data structure.

local_irq_save/restore while accessing local_paca->reentrant_args here
would be sufficient I think?

Yes, you are right.
I will also add preempt_{dis,en}able, which in most kernels will
compile out, but it will be kind of 'ready' if we ever decide to
support PREEMPT.

Thanks for the feedback!