powerpc/pseries: add RTAS work area allocator

powerpc/pseries: add RTAS work area allocator

Various pseries-specific RTAS functions take a temporary "work area"
parameter - a buffer in memory accessible to RTAS. Typically such
functions are passed the statically allocated rtas_data_buf buffer as
the argument. This buffer is protected by a global spinlock. So users
of rtas_data_buf cannot perform sleeping operations while accessing
the buffer.

Most RTAS functions that have a work area parameter can return a
status (-2/990x) that indicates that the caller should retry. Before
retrying, the caller may need to reschedule or sleep (see
rtas_busy_delay() for details). This combination of factors
leads to uncomfortable constructions like this:

	do {
		spin_lock(&rtas_data_buf_lock);
		rc = rtas_call(token, __pa(rtas_data_buf, ...);
		if (rc == 0) {
			/* parse or copy out rtas_data_buf contents */
		}
		spin_unlock(&rtas_data_buf_lock);
	} while (rtas_busy_delay(rc));

Another unfortunately common way of handling this is for callers to
blithely ignore the possibility of a -2/990x status and hope for the
best.

If users were allowed to perform blocking operations while owning a
work area, the programming model would become less tedious and
error-prone. Users could schedule away, sleep, or perform other
blocking operations without having to release and re-acquire
resources.

We could continue to use a single work area buffer, and convert
rtas_data_buf_lock to a mutex. But that would impose an unnecessarily
coarse serialization on all users. As awkward as the current design
is, it prevents longer running operations that need to repeatedly use
rtas_data_buf from blocking the progress of others.

There are more considerations. One is that while 4KB is fine for all
current in-kernel uses, some RTAS calls can take much smaller buffers,
and some (VPD, platform dumps) would likely benefit from larger
ones. Another is that at least one RTAS function (ibm,get-vpd)
has *two* work area parameters. And finally, we should expect the
number of work area users in the kernel to increase over time as we
introduce lockdown-compatible ABIs to replace less safe use cases
based on sys_rtas/librtas.

So a special-purpose allocator for RTAS work area buffers seems worth
trying.

Properties:

* The backing memory for the allocator is reserved early in boot in
  order to satisfy RTAS addressing requirements, and then managed with
  genalloc.
* Allocations can block, but they never fail (mempool-like).
* Prioritizes first-come, first-serve fairness over throughput.
* Early boot allocations before the allocator has been initialized are
  served via an internal static buffer.

Intended to replace rtas_data_buf. New code that needs RTAS work area
buffers should prefer this API.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230125-b4-powerpc-rtas-queue-v3-12-26929c8cce78@linux.ibm.com