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Subject[PATCH v2] x86/lib/copy_user_64.S: Handle 4-byte nocache copy
Date
Data corruption issues were observed in tests which initiated
a system crash/reset while accessing BTT devices. This problem
is reproducible.

The BTT driver calls pmem_rw_bytes() to update data in pmem
devices. This interface calls __copy_user_nocache(), which
uses non-temporal stores so that the stores to pmem are
persistent.

__copy_user_nocache() uses non-temporal stores when a request
size is 8 bytes or larger (and is aligned by 8 bytes). The
BTT driver updates the BTT map table, which entry size is
4 bytes. Therefore, updates to the map table entries remain
cached, and are not written to pmem after a crash.

Change __copy_user_nocache() to use non-temporal store when
a request size is 4 bytes. The change extends the current
byte-copy path for a less-than-8-bytes request, and does not
add any overhead to the regular path.

Also add comments to the code, and clarify the cases that
lead cache copy.

Reported-and-tested-by: Micah Parrish <micah.parrish@hpe.com>
Reported-and-tested-by: Brian Boylston <brian.boylston@hpe.com>
Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
---
v2:
- Add comments (Ingo Molnar).
- Make this patch as an individual patch since v2 debug changes
will not depend on this patch.
---
arch/x86/lib/copy_user_64.S | 74 +++++++++++++++++++++++++++++++++++--------
1 file changed, 61 insertions(+), 13 deletions(-)

diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
index 982ce34..1641327 100644
--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -232,17 +232,30 @@ ENDPROC(copy_user_enhanced_fast_string)

/*
* copy_user_nocache - Uncached memory copy with exception handling
- * This will force destination/source out of cache for more performance.
+ * This will force destination out of cache for more performance.
+ *
+ * Note: Cached memory copy is used when destination or size is not
+ * naturally aligned. That is:
+ * - Require 8-byte alignment when size is 8 bytes or larger.
+ * - Require 4-byte alignment when size is 4 bytes.
*/
ENTRY(__copy_user_nocache)
ASM_STAC
+
+ /* If size is less than 8 bytes, goto 4-byte copy */
cmpl $8,%edx
- jb 20f /* less then 8 bytes, go to byte copy loop */
+ jb 20f
+
+ /* If destination is not 8-byte aligned, "cache" copy to align it */
ALIGN_DESTINATION
+
+ /* Set 4x8-byte copy count and remainder */
movl %edx,%ecx
andl $63,%edx
shrl $6,%ecx
- jz 17f
+ jz 17f /* If count is 0, goto 8-byte copy */
+
+ /* Perform 4x8-byte nocache loop-copy */
1: movq (%rsi),%r8
2: movq 1*8(%rsi),%r9
3: movq 2*8(%rsi),%r10
@@ -263,26 +276,57 @@ ENTRY(__copy_user_nocache)
leaq 64(%rdi),%rdi
decl %ecx
jnz 1b
+
+ /* Set 8-byte copy count and remainder */
17: movl %edx,%ecx
andl $7,%edx
shrl $3,%ecx
- jz 20f
+ jz 20f /* If count is 0, goto 4-byte copy */
+
+ /* Perform 8-byte nocache loop-copy */
18: movq (%rsi),%r8
19: movnti %r8,(%rdi)
leaq 8(%rsi),%rsi
leaq 8(%rdi),%rdi
decl %ecx
jnz 18b
+
+ /* If no byte left, we're done */
20: andl %edx,%edx
- jz 23f
+ jz 26f
+
+ /* If destination is not 4-byte aligned, goto byte copy */
+ movl %edi,%ecx
+ andl $3,%ecx
+ jnz 23f
+
+ /* Set 4-byte copy count (1 or 0) and remainder */
movl %edx,%ecx
-21: movb (%rsi),%al
-22: movb %al,(%rdi)
+ andl $3,%edx
+ shrl $2,%ecx
+ jz 23f /* If count is 0, goto byte copy */
+
+ /* Perform 4-byte nocache copy */
+21: movl (%rsi),%r8d
+22: movnti %r8d,(%rdi)
+ leaq 4(%rsi),%rsi
+ leaq 4(%rdi),%rdi
+
+ /* If no byte left, we're done */
+ andl %edx,%edx
+ jz 26f
+
+ /* Perform byte "cache" loop-copy for the remainder */
+23: movl %edx,%ecx
+24: movb (%rsi),%al
+25: movb %al,(%rdi)
incq %rsi
incq %rdi
decl %ecx
- jnz 21b
-23: xorl %eax,%eax
+ jnz 24b
+
+ /* Finished copying; fence the prior stores */
+26: xorl %eax,%eax
ASM_CLAC
sfence
ret
@@ -290,11 +334,13 @@ ENTRY(__copy_user_nocache)
.section .fixup,"ax"
30: shll $6,%ecx
addl %ecx,%edx
- jmp 60f
+ jmp 70f
40: lea (%rdx,%rcx,8),%rdx
- jmp 60f
-50: movl %ecx,%edx
-60: sfence
+ jmp 70f
+50: lea (%rdx,%rcx,4),%rdx
+ jmp 70f
+60: movl %ecx,%edx
+70: sfence
jmp copy_user_handle_tail
.previous

@@ -318,4 +364,6 @@ ENTRY(__copy_user_nocache)
_ASM_EXTABLE(19b,40b)
_ASM_EXTABLE(21b,50b)
_ASM_EXTABLE(22b,50b)
+ _ASM_EXTABLE(24b,60b)
+ _ASM_EXTABLE(25b,60b)
ENDPROC(__copy_user_nocache)
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