[RFC PATCH v3 1/5] PCI: Add support for enforcing all MMIO BARs to be page aligned

[Yongji Xie]

When vfio passthrough a PCI device of which MMIO BARs
are smaller than PAGE_SIZE, guest will not handle the
mmio accesses to the BARs which leads to mmio emulations
in host.

This is because vfio will not allow to passthrough one
BAR's mmio page which may be shared with other BARs.

To solve this performance issue, this patch adds a kernel
parameter "pci=resource_page_aligned=on" to enforce
the alignment of all MMIO BARs to be at least PAGE_SIZE,
so that one BAR's mmio page would not be shared with other
BARs. We can also disable it through kernel parameter
"pci=resource_page_aligned=off".

For the default value of the parameter, we think it should be
arch-independent, so we add a macro
HAVE_PCI_DEFAULT_RESOURCES_PAGE_ALIGNED to change it. And we
define this macro to enable this parameter by default on PPC64
platform which can easily hit this performance issue because
its PAGE_SIZE is 64KB.

Note that the kernel parameter won't works if kernel doesn't do
resources reallocation.

Signed-off-by: Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx>
---
 Documentation/kernel-parameters.txt |    5 +++++
 arch/powerpc/include/asm/pci.h      |   11 +++++++++++
 drivers/pci/pci.c                   |   35 +++++++++++++++++++++++++++++++++++
 drivers/pci/pci.h                   |    8 +++++++-
 include/linux/pci.h                 |    4 ++++
 5 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 742f69d..3f2a7c9 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2857,6 +2857,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				PAGE_SIZE is used as alignment.
 				PCI-PCI bridge can be specified, if resource
 				windows need to be expanded.
+		resource_page_aligned=	Enable/disable enforcing the alignment
+				of all PCI devices' memory resources to be
+				at least PAGE_SIZE if resources reallocation
+				is done by kernel.
+				Format: { "on" | "off" }
 		ecrc=		Enable/disable PCIe ECRC (transaction layer
 				end-to-end CRC checking).
 				bios: Use BIOS/firmware settings. This is the
diff --git a/arch/powerpc/include/asm/pci.h b/arch/powerpc/include/asm/pci.h
index 3453bd8..2d2b3ef 100644
--- a/arch/powerpc/include/asm/pci.h
+++ b/arch/powerpc/include/asm/pci.h
@@ -136,6 +136,17 @@ extern pgprot_t	pci_phys_mem_access_prot(struct file *file,
 					 unsigned long pfn,
 					 unsigned long size,
 					 pgprot_t prot);
+#ifdef CONFIG_PPC64
+
+/* For PPC64, We enforce all PCI MMIO BARs to be page aligned
+ * by default. This would be helpful to improve performance
+ * when we passthrough a PCI device of which BARs are smaller
+ * than PAGE_SIZE(64KB). And we can use kernel parameter
+ * "pci=resource_page_aligned=off" to disable it.
+ */
+#define HAVE_PCI_DEFAULT_RESOURCES_PAGE_ALIGNED	1
+
+#endif
 
 #define HAVE_ARCH_PCI_RESOURCE_TO_USER
 extern void pci_resource_to_user(const struct pci_dev *dev, int bar,
diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 314db8c..7b21238 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -99,6 +99,9 @@ u8 pci_cache_line_size;
  */
 unsigned int pcibios_max_latency = 255;
 
+bool pci_resources_page_aligned =
+	IS_ENABLED(HAVE_PCI_DEFAULT_RESOURCES_PAGE_ALIGNED);
+
 /* If set, the PCIe ARI capability will not be used. */
 static bool pcie_ari_disabled;
 
@@ -4746,6 +4749,35 @@ static ssize_t pci_resource_alignment_store(struct bus_type *bus,
 BUS_ATTR(resource_alignment, 0644, pci_resource_alignment_show,
 					pci_resource_alignment_store);
 
+static void pci_resources_get_page_aligned(char *str)
+{
+	if (!strncmp(str, "off", 3))
+		pci_resources_page_aligned = false;
+	else if (!strncmp(str, "on", 2))
+		pci_resources_page_aligned = true;
+}
+
+/*
+ * This function checks whether PCI BARs' mmio page will be shared
+ * with other BARs.
+ */
+bool pci_resources_share_page(struct pci_dev *dev, int resno)
+{
+	struct resource *res = dev->resource + resno;
+
+	if (resource_size(res) >= PAGE_SIZE)
+		return false;
+	if (pci_resources_page_aligned && !(res->start & ~PAGE_MASK) &&
+	    res->flags & IORESOURCE_MEM) {
+		if (res->sibling)
+			return (res->sibling->start & ~PAGE_MASK);
+		else
+			return false;
+	}
+	return true;
+}
+EXPORT_SYMBOL_GPL(pci_resources_share_page);
+
 static int __init pci_resource_alignment_sysfs_init(void)
 {
 	return bus_create_file(&pci_bus_type,
@@ -4859,6 +4891,9 @@ static int __init pci_setup(char *str)
 			} else if (!strncmp(str, "resource_alignment=", 19)) {
 				pci_set_resource_alignment_param(str + 19,
 							strlen(str + 19));
+			} else if (!strncmp(str, "resource_page_aligned=",
+				   22)) {
+				pci_resources_get_page_aligned(str + 22);
 			} else if (!strncmp(str, "ecrc=", 5)) {
 				pcie_ecrc_get_policy(str + 5);
 			} else if (!strncmp(str, "hpiosize=", 9)) {
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index d390fc1..b9b333d 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -312,11 +312,17 @@ static inline resource_size_t pci_resource_alignment(struct pci_dev *dev,
 #ifdef CONFIG_PCI_IOV
 	int resno = res - dev->resource;
 
-	if (resno >= PCI_IOV_RESOURCES && resno <= PCI_IOV_RESOURCE_END)
+	if (resno >= PCI_IOV_RESOURCES && resno <= PCI_IOV_RESOURCE_END) {
+		if (pci_resources_page_aligned && res->flags & IORESOURCE_MEM)
+			return PAGE_ALIGN(pci_sriov_resource_alignment(dev,
+					  resno));
 		return pci_sriov_resource_alignment(dev, resno);
+	}
 #endif
 	if (dev->class >> 8  == PCI_CLASS_BRIDGE_CARDBUS)
 		return pci_cardbus_resource_alignment(res);
+	if (pci_resources_page_aligned && res->flags & IORESOURCE_MEM)
+		return PAGE_ALIGN(resource_alignment(res));
 	return resource_alignment(res);
 }
 
diff --git a/include/linux/pci.h b/include/linux/pci.h
index 6ae25aa..b640d65 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -1530,6 +1530,10 @@ static inline int pci_get_new_domain_nr(void) { return -ENOSYS; }
 	 (pci_resource_end((dev), (bar)) -		\
 	  pci_resource_start((dev), (bar)) + 1))
 
+extern bool pci_resources_page_aligned;
+
+bool pci_resources_share_page(struct pci_dev *dev, int resno);
+
 /* Similar to the helpers above, these manipulate per-pci_dev
  * driver-specific data.  They are really just a wrapper around
  * the generic device structure functions of these calls.
-- 
1.7.9.5