[PATCH V2] x86: Fix an issue with invalid ACPI NUMA config

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

The addition of support to read the NUMA node for a PCI card specified by
_PXM resulted in Martin's system not booting.   Looking at the ACPI tables
it seems that there are _PXM entries for the root ports, but no SRAT table.

The absence of the SRAT table results in dummy_numa_init() being called.
However, unlike on arm64, this doesn't result in numa_off being set.

When the PCI code later comes along and calls acpi_get_node() for any PCI
card below the root port, it navigates up the ACPI tree until it finds the
_PXM value in the root port. This value is then passed to
acpi_map_pxm_to_node().

As numa_off has not been set on x86 it tries to allocate a NUMA node, from
the unused set, without setting up all the infrastructure that would
normally accompany such a call.  We have not identified exactly which driver
is causing the subsequent hang for Martin.

If numa_off had been set, as it is in the equivalent flow on arm64, then
acpi_map_pxm_to_node() would return NUMA_NO_NODE, which is what we want to
happen.

It is invalid under the ACPI spec to specify new NUMA nodes using _PXM if
they have no presence in SRAT. Thus the simplest fix is to set numa_off when
NUMA support is disabled due to an invalid SRAT (here not present at all).

I do not have easy access to appropriate x86 NUMA systems so would
appreciate some testing of this one!

Known problem boards setups:

AMD Ryzen Threadripper 2950X on ASROCK X399 TAICHI
MSI X399 SLI PLUS (probably - not confirmed yet)

The PCI patch has been reverted, so this fix is not critical.

Reported-by: Martin Hundebøll <martin@geanix.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Fixes: bad7dcd94f39 ("ACPI/PCI: Pay attention to device-specific _PXM node values")
---
Changes since V1:
* Update commit message as suggested by Bjorn Helgaas.
* No functional changes.

 arch/x86/mm/numa.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/mm/numa.c b/arch/x86/mm/numa.c
index 1308f5408bf7..ce1182f953ff 100644
--- a/arch/x86/mm/numa.c
+++ b/arch/x86/mm/numa.c
@@ -695,6 +695,8 @@ static int __init dummy_numa_init(void)
 	node_set(0, numa_nodes_parsed);
 	numa_add_memblk(0, 0, PFN_PHYS(max_pfn));
 
+	numa_off = true;
+
 	return 0;
 }
 
-- 
2.19.1