[PATCH AUTOSEL 6.18] PCI: hv: Set default NUMA node to 0 for devices without affinity info

[Sasha Levin <sashal@kernel.org>]

From: Long Li <longli@microsoft.com>

[ Upstream commit 7b3b1e5a87b2f5e35c52b5386d7c327be869454f ]

When hv_pci_assign_numa_node() processes a device that does not have
HV_PCI_DEVICE_FLAG_NUMA_AFFINITY set or has an out-of-range
virtual_numa_node, the device NUMA node is left unset. On x86_64,
the uninitialized default happens to be 0, but on ARM64 it is
NUMA_NO_NODE (-1).

Tests show that when no NUMA information is available from the Hyper-V
host, devices perform best when assigned to node 0. With NUMA_NO_NODE
the kernel may spread work across NUMA nodes, which degrades
performance on Hyper-V, particularly for high-throughput devices like
MANA.

Always set the device NUMA node to 0 before the conditional NUMA
affinity check, so that devices get a performant default when the host
provides no NUMA information, and behavior is consistent on both
x86_64 and ARM64.

Fixes: 999dd956d838 ("PCI: hv: Add support for protocol 1.3 and support PCI_BUS_RELATIONS2")
Signed-off-by: Long Li <longli@microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

Error: Failed to generate final synthesis

 drivers/pci/controller/pci-hyperv.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
index 146b43981b278..28b1572974879 100644
--- a/drivers/pci/controller/pci-hyperv.c
+++ b/drivers/pci/controller/pci-hyperv.c
@@ -2486,6 +2486,14 @@ static void hv_pci_assign_numa_node(struct hv_pcibus_device *hbus)
 		if (!hv_dev)
 			continue;
 
+		/*
+		 * If the Hyper-V host doesn't provide a NUMA node for the
+		 * device, default to node 0. With NUMA_NO_NODE the kernel
+		 * may spread work across NUMA nodes, which degrades
+		 * performance on Hyper-V.
+		 */
+		set_dev_node(&dev->dev, 0);
+
 		if (hv_dev->desc.flags & HV_PCI_DEVICE_FLAG_NUMA_AFFINITY &&
 		    hv_dev->desc.virtual_numa_node < num_possible_nodes())
 			/*
-- 
2.53.0