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SubjectRe: [RFC Patch] use MTRR for write combining if PAT is not available
Date
Jan Beulich wrote:
> >>> Thomas Schlichter <thomas.schlichter@web.de> 13.10.09 23:29 >>>
> >No, at least the comments in mtrr_add and mtrr_check state that it is just
> >required that phys_addr and size are multiple of PAGE_SIZE. And I'm not
> > sure if it is always safe to round these up/down to the next PAGE
> > boundary. If it is not, maybe it is better to fail...
>
> That function isn't the limiting factor, generic_validate_add_page() is
> what you need to look at (plus the spec on how the MTRR ranges are
> calculated by the CPU from the base/mask register pairs).

Yes, you are right. Sorry for not looking into generic_validate_add_page()
before. Thank you for showing!

I added a function mtrr_add_unaligned() that tries to create as many MTRR
entries as necessary, beginning with the biggest regions. It does not check
the return values of each mtrr_add(), nor does it return the indexes of the
created MTRR entries. So it seems to be only useful with increment=false. Or
do you have a better idea?

Kind regards,
Thomas
From 3708f0f4c729de32445ba13ab16b6268920af0bb Mon Sep 17 00:00:00 2001
From: Thomas Schlichter <thomas.schlichter@web.de>
Date: Wed, 14 Oct 2009 19:25:33 +0200
Subject: [PATCH 1/2] Add new mtrr_add_unaligned function

This function creates multiple MTRR entries for unaligned memory regions.

Signed-off-by: Thomas Schlichter <thomas.schlichter@web.de>
---
arch/x86/include/asm/mtrr.h | 6 ++++++
arch/x86/kernel/cpu/mtrr/main.c | 25 +++++++++++++++++++++++++
2 files changed, 31 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
index 4365ffd..0ad8e68 100644
--- a/arch/x86/include/asm/mtrr.h
+++ b/arch/x86/include/asm/mtrr.h
@@ -116,6 +116,8 @@ extern int mtrr_add(unsigned long base, unsigned long size,
unsigned int type, bool increment);
extern int mtrr_add_page(unsigned long base, unsigned long size,
unsigned int type, bool increment);
+extern void mtrr_add_unaligned(unsigned long base, unsigned long size,
+ unsigned int type, bool increment);
extern int mtrr_del(int reg, unsigned long base, unsigned long size);
extern int mtrr_del_page(int reg, unsigned long base, unsigned long size);
extern void mtrr_centaur_report_mcr(int mcr, u32 lo, u32 hi);
@@ -146,6 +148,10 @@ static inline int mtrr_add_page(unsigned long base, unsigned long size,
{
return -ENODEV;
}
+static inline void mtrr_add_unaligned(unsigned long base, unsigned long size,
+ unsigned int type, bool increment)
+{
+}
static inline int mtrr_del(int reg, unsigned long base, unsigned long size)
{
return -ENODEV;
diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
index 84e83de..7417ebb 100644
--- a/arch/x86/kernel/cpu/mtrr/main.c
+++ b/arch/x86/kernel/cpu/mtrr/main.c
@@ -487,6 +487,31 @@ int mtrr_add(unsigned long base, unsigned long size, unsigned int type,
}
EXPORT_SYMBOL(mtrr_add);

+void mtrr_add_unaligned(unsigned long base, unsigned long size,
+ unsigned int type, bool increment)
+{
+ unsigned long ptr1, ptr2, end = base + size;
+
+ // round down size to next power ot two
+ size = __rounddown_pow_of_two(size);
+
+ // accordingly align pointers
+ ptr1 = ptr2 = (base + size - 1) & ~(size - 1);
+
+ while (size >= PAGE_SIZE) {
+ if (ptr1 + size <= end) {
+ mtrr_add(ptr1, size, type, increment);
+ ptr1 += size;
+ }
+ if (base + size <= ptr2) {
+ ptr2 -= size;
+ mtrr_add(ptr2, size, type, increment);
+ }
+ size >>= 1;
+ }
+}
+EXPORT_SYMBOL(mtrr_add_unaligned);
+
/**
* mtrr_del_page - delete a memory type region
* @reg: Register returned by mtrr_add
--
1.6.5
From 3533b9e66c5144844a0b0864d0f57f43d57aea1a Mon Sep 17 00:00:00 2001
From: Thomas Schlichter <thomas.schlichter@web.de>
Date: Thu, 8 Oct 2009 21:24:07 +0200
Subject: [PATCH 2/2] Use MTRR for write combining mmap/ioremap if PAT is not available

X.org uses libpciaccess which tries to mmap with write combining enabled via
/sys/bus/pci/devices/*/resource0_wc. Currently, when PAT is not enabled, we
fall back to uncached mmap. Then libpciaccess thinks it succeeded mapping
with write combining anabled and does not set up suited MTRR entries. ;-(

So when falling back to uncached mapping, we better try to set up MTRR
entries automatically. To match this modified PCI mmap behavior, also
ioremap_wc and set_memory_wc are adjusted.

Signed-off-by: Thomas Schlichter <thomas.schlichter@web.de>
---
arch/x86/mm/ioremap.c | 15 ++++++++++-----
arch/x86/mm/pageattr.c | 10 ++++++++--
arch/x86/pci/i386.c | 6 ++++++
3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index 334e63c..abe40fa 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -21,6 +21,7 @@
#include <asm/tlbflush.h>
#include <asm/pgalloc.h>
#include <asm/pat.h>
+#include <asm/mtrr.h>

#include "physaddr.h"

@@ -268,11 +269,15 @@ EXPORT_SYMBOL(ioremap_nocache);
*/
void __iomem *ioremap_wc(resource_size_t phys_addr, unsigned long size)
{
- if (pat_enabled)
- return __ioremap_caller(phys_addr, size, _PAGE_CACHE_WC,
- __builtin_return_address(0));
- else
- return ioremap_nocache(phys_addr, size);
+ if (!pat_enabled) {
+ void __iomem *ret = ioremap_nocache(phys_addr, size);
+ if (ret)
+ mtrr_add_unaligned(phys_addr, size,
+ MTRR_TYPE_WRCOMB, false);
+ return ret;
+ }
+ return __ioremap_caller(phys_addr, size, _PAGE_CACHE_WC,
+ __builtin_return_address(0));
}
EXPORT_SYMBOL(ioremap_wc);

diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
index dd38bfb..c25f697 100644
--- a/arch/x86/mm/pageattr.c
+++ b/arch/x86/mm/pageattr.c
@@ -23,6 +23,7 @@
#include <asm/pgalloc.h>
#include <asm/proto.h>
#include <asm/pat.h>
+#include <asm/mtrr.h>

/*
* The current flushing context - we pass it instead of 5 arguments:
@@ -1010,8 +1011,13 @@ int set_memory_wc(unsigned long addr, int numpages)
{
int ret;

- if (!pat_enabled)
- return set_memory_uc(addr, numpages);
+ if (!pat_enabled) {
+ ret = set_memory_uc(addr, numpages);
+ if (!ret)
+ mtrr_add_unaligned(__pa(addr), numpages * PAGE_SIZE,
+ MTRR_TYPE_WRCOMB, false);
+ return ret;
+ }

ret = reserve_memtype(__pa(addr), __pa(addr) + numpages * PAGE_SIZE,
_PAGE_CACHE_WC, NULL);
diff --git a/arch/x86/pci/i386.c b/arch/x86/pci/i386.c
index b22d13b..8379e9b 100644
--- a/arch/x86/pci/i386.c
+++ b/arch/x86/pci/i386.c
@@ -33,6 +33,7 @@
#include <linux/bootmem.h>

#include <asm/pat.h>
+#include <asm/mtrr.h>
#include <asm/e820.h>
#include <asm/pci_x86.h>
#include <asm/io_apic.h>
@@ -301,5 +302,10 @@ int pci_mmap_page_range(struct pci_dev *dev, struct vm_area_struct *vma,

vma->vm_ops = &pci_mmap_ops;

+ if (!pat_enabled && write_combine)
+ mtrr_add_unaligned(vma->vm_pgoff << PAGE_SHIFT,
+ vma->vm_end - vma->vm_start,
+ MTRR_TYPE_WRCOMB, false);
+
return 0;
}
--
1.6.5
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