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

From: Yongji Xie
Date: Fri Jan 29 2016 - 05:38:19 EST

On 2016/1/29 6:46, Alex Williamson wrote:
On Fri, 2016-01-15 at 15:06 +0800, Yongji Xie wrote:
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
For the default value of the parameter, we think it should be
arch-independent, so we add a macro
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.
And where do you account for this so that we know whether it's really in

We can check the flag PCI_PROBE_ONLY to know whether kernel do
resources reallocation. Then we know if the kernel parameter is really
in effect.

enum {
/* Force re-assigning all resources (ignore firmware
* setup completely)

/* Re-assign all bus numbers */
PCI_REASSIGN_ALL_BUS = 0x00000002,

/* Do not try to assign, just use existing setup */
---> PCI_PROBE_ONLY = 0x00000004,

And I will add this to commit log.

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.
+ */
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 =
I don't think this is proper use of IS_ENABLED, which seems to be
targeted at CONFIG_ type options. You could define this as that in an
arch Kconfig.

Is it better that we define this as a pci Kconfig and select it in arch Kconfig?

/* 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,
+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;

How about pci_resources_parse_page_aligned_param()?

+ * 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;
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);
Doesn't this seem rather redundant with the option right above it,
resource_alignment=? Why not modify that to support syntax where all
devices get the same alignment?

The kernel option will be used to do two things.

Firstly, the option can be used to enable all devices to be page aligned.

Secondly, we can use the option to disable it when the Kconfig option
mentioned above enable all devices to be page aligned by default.

We can easily modify this option "resource_alignment=" to do the first
thing. But I didn't find a proper way to modify it to do the second thing.

} 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,
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);
+ }
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);

Since we already have resource_alignment=, shouldn't we already have the
code in place to re-align?

Yes, this code can do the re-aligning. But we can't reuse the code because
it re-align device's bars by changing their sizes, which can potentially break
some drivers.

I'm thinking if we can use IORESOURCE_STARTALIGN for this. Thanks.

Yongji Xie