Subject: Re: [PATCH] DMA 4GB boundary protection

From: Jake Moilanen <[email-masked]>
Date: Wed, 28 Mar 2007 13:17:22 -0500

> > +	/*
> > +	 * DMA cannot cross 4 GB boundary.  Mark last entry of each 4
> > +	 * GB chunk as reserved.
> > +	 */
> > +	if (protect4gb) {
> > +		entries_per_4g = 0x100000000l >> IOMMU_PAGE_SHIFT;
> > +
> > +		/* Mark the last bit before a 4GB boundary as used */
> > +		start_index = (tbl->it_offset << IOMMU_PAGE_SHIFT) >> 32;
> > +		start_index |= (entries_per_4g - 1);
> 
> This looks broken.
> 
> The idea is to make start_index the last page before the first 4GB
> boundary after it_offset. If that happens to be beyond end_index the
> for loop below will never run. If it's below that, every last page in
> the 4GB ranges will be marked in the loop. This will work even if the
> table starts at i.e. 2GB and goes until 10GB.
> 
> With the first line above, your start_index will always be 0xfffff
> (unless the offset is waay up there in the address space).
> 
> The logic I had was:
> 
> 	start_index = tbl->it_offset | (entries_per_4g - 1);                                                                                   
> 
> This is also broken, since it doesn't consider it_offset in the loop
> below. That was my bad, and I guess was what you tried to fix above.
> 
> What you really want is:
> 	
> 	start_index = tbl->it_offset | (entries_per_4g - 1);                                                                                   
> 	start_index -= tbl->it_offset;
> 
> 	end_index = tbl->it_size;

Yup.

> Say that it_offset is at 3GB, with 4KB pages that means the value is
> 0xc0000. entries_per_4g is 0x100000, i.e. the logic becomes: 0xc0000 |
> 0xfffff = 0xfffff (- 0xc0000 = 0x3ffff), which indeed is the last page
> before 4GB.
> 
> If it_offset is at 9GB, i.e. 0x240000, then we get start_index at 0x2fffff
> (- 0x240000 = 0xbffff) , i.e. yet again last page before the 12GB wrap.

One more try.

There are many adapters which can not handle DMAing acrosss any 4 GB
boundary.  For instance the latest Emulex adapters.  

This normally is not an issue as firmware gives dma-windows under
4gigs.  However, some of the new System-P boxes have dma-windows above
4gigs, and this present a problem.

During initialization of the IOMMU tables, the last entry at each 4GB
boundary is marked as used.  Thus no mappings can cross the boundary.
If a table ends at a 4GB boundary, the entry is not marked as used.

A boot option to remove this 4GB protection is given w/ protect4gb=off.
This exposes the potential issue for driver and hardware development
purposes.

Signed-off-by: Jake Moilanen <[email-masked]>
---
 arch/powerpc/kernel/iommu.c |   35 ++++++++++++++++++++++++++++++++++-
 1 files changed, 34 insertions(+), 1 deletion(-)

Index: powerpc/arch/powerpc/kernel/iommu.c
===================================================================
--- powerpc.orig/arch/powerpc/kernel/iommu.c
+++ powerpc/arch/powerpc/kernel/iommu.c
@@ -47,6 +47,8 @@ static int novmerge = 0;
 static int novmerge = 1;
 #endif
 
+static int protect4gb = 1;
+
 static inline unsigned long iommu_num_pages(unsigned long vaddr,
 					    unsigned long slen)
 {
@@ -58,6 +60,16 @@ static inline unsigned long iommu_num_pa
 	return npages;
 }
 
+static int __init setup_protect4gb(char *str)
+{
+	if (strcmp(str, "on") == 0)
+		protect4gb = 1;
+	else if (strcmp(str, "off") == 0)
+		protect4gb = 0;
+
+	return 1;
+}
+
 static int __init setup_iommu(char *str)
 {
 	if (!strcmp(str, "novmerge"))
@@ -67,6 +79,7 @@ static int __init setup_iommu(char *str)
 	return 1;
 }
 
+__setup("protect4gb=", setup_protect4gb);
 __setup("iommu=", setup_iommu);
 
 static unsigned long iommu_range_alloc(struct iommu_table *tbl,
@@ -429,6 +442,9 @@ void iommu_unmap_sg(struct iommu_table *
 struct iommu_table *iommu_init_table(struct iommu_table *tbl, int nid)
 {
 	unsigned long sz;
+	unsigned long start_index, end_index;
+	unsigned long entries_per_4g;
+	unsigned long index;
 	static int welcomed = 0;
 	struct page *page;
 
@@ -450,7 +466,7 @@ struct iommu_table *iommu_init_table(str
 
 #ifdef CONFIG_CRASH_DUMP
 	if (ppc_md.tce_get) {
-		unsigned long index, tceval;
+		unsigned long tceval;
 		unsigned long tcecount = 0;
 
 		/*
@@ -480,6 +496,23 @@ struct iommu_table *iommu_init_table(str
 	ppc_md.tce_free(tbl, tbl->it_offset, tbl->it_size);
 #endif
 
+	/*
+	 * DMA cannot cross 4 GB boundary.  Mark last entry of each 4
+	 * GB chunk as reserved.
+	 */
+	if (protect4gb) {
+		entries_per_4g = 0x100000000l >> IOMMU_PAGE_SHIFT;
+
+		/* Mark the last bit before a 4GB boundary as used */
+		start_index = tbl->it_offset | (entries_per_4g - 1);
+		start_index -= tbl->it_offset;
+
+		end_index = tbl->it_size;
+
+		for (index = start_index; index < end_index - 1; index +=
entries_per_4g)
+			__set_bit(index, tbl->it_map);
+	}
+
 	if (!welcomed) {
 		printk(KERN_INFO "IOMMU table initialized, virtual merging %s\n",
 		       novmerge ? "disabled" : "enabled");