linux-kernel - [PATCH 6/6] PCI: align small (<4k) BARs

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Message-ID: <20240709133610.1089420-7-stewart.hildebrand@amd.com>
Date: Tue, 9 Jul 2024 09:36:03 -0400
From: Stewart Hildebrand <stewart.hildebrand@....com>
To: Bjorn Helgaas <bhelgaas@...gle.com>
CC: Stewart Hildebrand <stewart.hildebrand@....com>,
	<linux-pci@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: [PATCH 6/6] PCI: align small (<4k) BARs

Issues observed when small (<4k) BARs are not 4k aligned are:

1. Devices to be passed through (to e.g. a Xen HVM guest) with small
(<4k) BARs require each memory BAR to be page aligned. Currently, the
only way to guarantee this alignment from a user perspective is to fake
the size of the BARs using the pci=resource_alignment= option. This is a
bad user experience, and faking the BAR size is not always desirable.
See the comment in drivers/pci/pci.c:pci_request_resource_alignment()
for further discussion.

2. Devices with multiple small (<4k) BARs could have the MSI-X tables
located in one of its small (<4k) BARs. This may lead to the MSI-X
tables being mapped in the same 4k region as other data. The PCIe 6.1
specification (section 7.7.2 MSI-X Capability and Table Structure) says
we probably shouldn't do that.

To improve the user experience, and increase conformance to PCIe spec,
set the default minimum resource alignment of memory BARs to 4k. Choose
4k (rather than PAGE_SIZE) for the alignment value in the common code,
since that is the value called out in the PCIe 6.1 spec, section 7.7.2.
The new default alignment may be overridden by arches by implementing
pcibios_default_alignment(), or by the user with the
pci=resource_alignment= option.

Signed-off-by: Stewart Hildebrand <stewart.hildebrand@....com>
---
Preparatory patches in this series are prerequisites to this patch.
---
 drivers/pci/pci.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 9f7894538334..e7b648304383 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -6453,7 +6453,12 @@ struct pci_dev __weak *pci_real_dma_dev(struct pci_dev *dev)

 resource_size_t __weak pcibios_default_alignment(void)
 {
-	return 0;
+	/*
+	 * Avoid MSI-X tables being mapped in the same 4k region as other data
+	 * according to PCIe 6.1 specification section 7.7.2 MSI-X Capability
+	 * and Table Structure.
+	 */
+	return 4 * 1024;
 }

 /*
-- 
2.45.2