[PATCH]intel-iommu batched iotlb flushes

From: mark gross
Date: Mon Feb 11 2008 - 17:43:32 EST

The intel-iommu hardware requires a polling operation to flush IOTLB
PTE's after an unmap operation. Through some TSC instrumentation of a
netperf UDP stream with small packets test case it was seen that the
flush operations where sucking up to 16% of the CPU time doing
iommu_flush_iotlb's

The following patch batches the IOTLB flushes removing most of the
overhead in flushing the IOTLB's. It works by building a list of to be
released IOVA's that is iterated over when a timer goes off or when a
high water mark is reached.

The wrinkle this has is that the memory protection and page fault
warnings from errant DMA operations is somewhat reduced, hence a kernel
parameter is added to revert back to the "strict" page flush / unmap
behavior.

The hole is the following scenarios:
do many map_signal operations, do some unmap_signals, reuse a recently
unmapped page, <errant DMA hardware sneaks through and steps on reused
memory>

Or: you have rouge hardware using DMA's to look at pages: do many
map_signal's, do many unmap_singles, reuse some unmapped pages :
<rouge hardware looks at reused page>

Note : these holes are very hard to get too, as the IOTLB is small and
only the PTE's still in the IOTLB can be accessed through this
mechanism.

Its recommended that strict is used when developing drivers that do DMA
operations to catch bugs early. For production code where performance
is desired running with the batched IOTLB flushing is a good way to
go.


--mgross


Signed-off-by: <mgross@xxxxxxxxxxxxxxx>



Index: linux-2.6.24-mm1/drivers/pci/intel-iommu.c
===================================================================
--- linux-2.6.24-mm1.orig/drivers/pci/intel-iommu.c 2008-02-07 13:03:10.000000000 -0800
+++ linux-2.6.24-mm1/drivers/pci/intel-iommu.c 2008-02-11 10:38:49.000000000 -0800
@@ -21,6 +21,7 @@

#include <linux/init.h>
#include <linux/bitmap.h>
+#include <linux/debugfs.h>
#include <linux/slab.h>
#include <linux/irq.h>
#include <linux/interrupt.h>
@@ -30,6 +31,7 @@
#include <linux/dmar.h>
#include <linux/dma-mapping.h>
#include <linux/mempool.h>
+#include <linux/timer.h>
#include "iova.h"
#include "intel-iommu.h"
#include <asm/proto.h> /* force_iommu in this header in x86-64*/
@@ -50,11 +52,39 @@

#define DOMAIN_MAX_ADDR(gaw) ((((u64)1) << gaw) - 1)

+
+static void flush_unmaps_timeout(unsigned long data);
+
+static struct timer_list unmap_timer =
+ TIMER_INITIALIZER(flush_unmaps_timeout, 0, 0);
+
+struct unmap_list {
+ struct list_head list;
+ struct dmar_domain *domain;
+ struct iova *iova;
+};
+
+static struct intel_iommu *g_iommus;
+/* bitmap for indexing intel_iommus */
+static unsigned long *g_iommus_to_flush;
+static int g_num_of_iommus;
+
+static DEFINE_SPINLOCK(async_umap_flush_lock);
+static LIST_HEAD(unmaps_to_do);
+
+static int timer_on;
+static long list_size;
+static int high_watermark;
+
+static struct dentry *intel_iommu_debug, *debug;
+
+
static void domain_remove_dev_info(struct dmar_domain *domain);

static int dmar_disabled;
static int __initdata dmar_map_gfx = 1;
static int dmar_forcedac;
+static int intel_iommu_strict;

#define DUMMY_DEVICE_DOMAIN_INFO ((struct device_domain_info *)(-1))
static DEFINE_SPINLOCK(device_domain_lock);
@@ -73,9 +103,13 @@
printk(KERN_INFO
"Intel-IOMMU: disable GFX device mapping\n");
} else if (!strncmp(str, "forcedac", 8)) {
- printk (KERN_INFO
+ printk(KERN_INFO
"Intel-IOMMU: Forcing DAC for PCI devices\n");
dmar_forcedac = 1;
+ } else if (!strncmp(str, "strict", 8)) {
+ printk(KERN_INFO
+ "Intel-IOMMU: disable bached IOTLB flush\n");
+ intel_iommu_strict = 1;
}

str += strcspn(str, ",");
@@ -965,17 +999,13 @@
set_bit(0, iommu->domain_ids);
return 0;
}
-
-static struct intel_iommu *alloc_iommu(struct dmar_drhd_unit *drhd)
+static struct intel_iommu *alloc_iommu(struct intel_iommu *iommu,
+ struct dmar_drhd_unit *drhd)
{
- struct intel_iommu *iommu;
int ret;
int map_size;
u32 ver;

- iommu = kzalloc(sizeof(*iommu), GFP_KERNEL);
- if (!iommu)
- return NULL;
iommu->reg = ioremap(drhd->reg_base_addr, PAGE_SIZE_4K);
if (!iommu->reg) {
printk(KERN_ERR "IOMMU: can't map the region\n");
@@ -1396,7 +1426,7 @@
int index;

while (dev) {
- for (index = 0; index < cnt; index ++)
+ for (index = 0; index < cnt; index++)
if (dev == devices[index])
return 1;

@@ -1661,7 +1691,7 @@
struct dmar_rmrr_unit *rmrr;
struct pci_dev *pdev;
struct intel_iommu *iommu;
- int ret, unit = 0;
+ int nlongs, i, ret, unit = 0;

/*
* for each drhd
@@ -1672,7 +1702,29 @@
for_each_drhd_unit(drhd) {
if (drhd->ignored)
continue;
- iommu = alloc_iommu(drhd);
+ g_num_of_iommus++;
+ }
+
+ nlongs = BITS_TO_LONGS(g_num_of_iommus);
+ g_iommus_to_flush = kzalloc(nlongs * sizeof(unsigned long), GFP_KERNEL);
+ if (!g_iommus_to_flush) {
+ printk(KERN_ERR "Allocating bitmap array failed\n");
+ return -ENOMEM;
+ }
+
+ g_iommus = kzalloc(g_num_of_iommus * sizeof(*iommu), GFP_KERNEL);
+ if (!g_iommus) {
+ kfree(g_iommus_to_flush);
+ ret = -ENOMEM;
+ goto error;
+ }
+
+ i = 0;
+ for_each_drhd_unit(drhd) {
+ if (drhd->ignored)
+ continue;
+ iommu = alloc_iommu(&g_iommus[i], drhd);
+ i++;
if (!iommu) {
ret = -ENOMEM;
goto error;
@@ -1705,7 +1757,6 @@
* endfor
*/
for_each_rmrr_units(rmrr) {
- int i;
for (i = 0; i < rmrr->devices_cnt; i++) {
pdev = rmrr->devices[i];
/* some BIOS lists non-exist devices in DMAR table */
@@ -1909,6 +1960,54 @@
return 0;
}

+static void flush_unmaps(void)
+{
+ struct iova *node, *n;
+ unsigned long flags;
+ int i;
+
+ spin_lock_irqsave(&async_umap_flush_lock, flags);
+ timer_on = 0;
+
+ /*just flush them all*/
+ for (i = 0; i < g_num_of_iommus; i++) {
+ if (test_and_clear_bit(i, g_iommus_to_flush))
+ iommu_flush_iotlb_global(&g_iommus[i], 0);
+ }
+
+ list_for_each_entry_safe(node, n, &unmaps_to_do, list) {
+ /* free iova */
+ list_del(&node->list);
+ __free_iova(&((struct dmar_domain *)node->dmar)->iovad, node);
+
+ }
+ list_size = 0;
+ spin_unlock_irqrestore(&async_umap_flush_lock, flags);
+}
+
+static void flush_unmaps_timeout(unsigned long data)
+{
+ flush_unmaps();
+}
+
+static void add_unmap(struct dmar_domain *dom, struct iova *iova)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&async_umap_flush_lock, flags);
+ iova->dmar = dom;
+ list_add(&iova->list, &unmaps_to_do);
+ set_bit((dom->iommu - g_iommus), g_iommus_to_flush);
+
+ if (!timer_on) {
+ unmap_timer.expires = jiffies + msecs_to_jiffies(10);
+ mod_timer(&unmap_timer, unmap_timer.expires);
+ timer_on = 1;
+ }
+ list_size++;
+ spin_unlock_irqrestore(&async_umap_flush_lock, flags);
+}
+
static void intel_unmap_single(struct device *dev, dma_addr_t dev_addr,
size_t size, int dir)
{
@@ -1936,13 +2035,21 @@
dma_pte_clear_range(domain, start_addr, start_addr + size);
/* free page tables */
dma_pte_free_pagetable(domain, start_addr, start_addr + size);
-
- if (iommu_flush_iotlb_psi(domain->iommu, domain->id, start_addr,
- size >> PAGE_SHIFT_4K, 0))
- iommu_flush_write_buffer(domain->iommu);
-
- /* free iova */
- __free_iova(&domain->iovad, iova);
+ if (intel_iommu_strict) {
+ if (iommu_flush_iotlb_psi(domain->iommu,
+ domain->id, start_addr, size >> PAGE_SHIFT_4K, 0))
+ iommu_flush_write_buffer(domain->iommu);
+ /* free iova */
+ __free_iova(&domain->iovad, iova);
+ } else {
+ add_unmap(domain, iova);
+ /*
+ * queue up the release of the unmap to save the 1/6th of the
+ * cpu used up by the iotlb flush operation...
+ */
+ if (list_size > high_watermark)
+ flush_unmaps();
+ }
}

static void * intel_alloc_coherent(struct device *hwdev, size_t size,
@@ -2266,6 +2373,10 @@
if (dmar_table_init())
return -ENODEV;

+ high_watermark = 250;
+ intel_iommu_debug = debugfs_create_dir("intel_iommu", NULL);
+ debug = debugfs_create_u32("high_watermark", S_IWUGO | S_IRUGO,
+ intel_iommu_debug, &high_watermark);
iommu_init_mempool();
dmar_init_reserved_ranges();

@@ -2281,6 +2392,7 @@
printk(KERN_INFO
"PCI-DMA: Intel(R) Virtualization Technology for Directed I/O\n");

+ init_timer(&unmap_timer);
force_iommu = 1;
dma_ops = &intel_dma_ops;
return 0;
Index: linux-2.6.24-mm1/drivers/pci/iova.h
===================================================================
--- linux-2.6.24-mm1.orig/drivers/pci/iova.h 2008-02-07 13:03:10.000000000 -0800
+++ linux-2.6.24-mm1/drivers/pci/iova.h 2008-02-07 13:06:28.000000000 -0800
@@ -23,6 +23,8 @@
struct rb_node node;
unsigned long pfn_hi; /* IOMMU dish out addr hi */
unsigned long pfn_lo; /* IOMMU dish out addr lo */
+ struct list_head list;
+ void *dmar;
};

/* holds all the iova translations for a domain */
Index: linux-2.6.24-mm1/Documentation/kernel-parameters.txt
===================================================================
--- linux-2.6.24-mm1.orig/Documentation/kernel-parameters.txt 2008-02-11 13:44:23.000000000 -0800
+++ linux-2.6.24-mm1/Documentation/kernel-parameters.txt 2008-02-11 14:23:37.000000000 -0800
@@ -822,6 +822,10 @@
than 32 bit addressing. The default is to look
for translation below 32 bit and if not available
then look in the higher range.
+ strict [Default Off]
+ With this option on every umap_signle will
+ result in a hardware IOTLB flush opperation as
+ opposed to batching them for performance.

io_delay= [X86-32,X86-64] I/O delay method
0x80
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