path: root/Documentation/x86
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authorNick Desaulniers <>2020-03-26 17:09:51 -0700
committerJonathan Corbet <>2020-03-27 10:50:01 -0600
commitabcb1e021ae5a36374c635eeaba5cec733169b78 (patch)
tree9a227dcb9b995a304d562649f246d982e7e7ebdc /Documentation/x86
parent19e91e543c82c5d7c5f0f1820ed60af1e88956e6 (diff)
Documentation: x86: exception-tables: document CONFIG_BUILDTIME_TABLE_SORT
Provide more information about __ex_table sorting post link. The exception tables and fixup tables use a commonly recurring pattern in the kernel of storing the address of labels as date in custom ELF sections, then finding these sections, iterating elements within them, and possibly revisiting them or modifying the data at these addresses. Sorting readonly arrays to minimize runtime penalties is quite clever. Suggested-by: H. Peter Anvin <> Signed-off-by: Nick Desaulniers <> Link: Signed-off-by: Jonathan Corbet <>
Diffstat (limited to 'Documentation/x86')
1 files changed, 14 insertions, 0 deletions
diff --git a/Documentation/x86/exception-tables.rst b/Documentation/x86/exception-tables.rst
index ed6d4b0cf62c..81a393867f10 100644
--- a/Documentation/x86/exception-tables.rst
+++ b/Documentation/x86/exception-tables.rst
@@ -257,6 +257,9 @@ the fault, in our case the actual value is c0199ff5:
the original assembly code: > 3: movl $-14,%eax
and linked in vmlinux : > c0199ff5 <.fixup+10b5> movl $0xfffffff2,%eax
+If the fixup was able to handle the exception, control flow may be returned
+to the instruction after the one that triggered the fault, ie. local label 2b.
The assembly code::
> .section __ex_table,"a"
@@ -344,3 +347,14 @@ pointer which points to one of:
it as special.
More functions can easily be added.
+CONFIG_BUILDTIME_TABLE_SORT allows the __ex_table section to be sorted post
+link of the kernel image, via a host utility scripts/sorttable. It will set the
+symbol main_extable_sort_needed to 0, avoiding sorting the __ex_table section
+at boot time. With the exception table sorted, at runtime when an exception
+occurs we can quickly lookup the __ex_table entry via binary search.
+This is not just a boot time optimization, some architectures require this
+table to be sorted in order to handle exceptions relatively early in the boot
+process. For example, i386 makes use of this form of exception handling before
+paging support is even enabled!