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Date:   Mon, 23 Mar 2020 16:22:14 -0700
Message-Id: <20200323232214.24939-1-ndesaulniers@google.com>
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Subject: [PATCH] Documentation: x86: exception-tables: document CONFIG_BUILDTIME_TABLE_SORT
From:   Nick Desaulniers <ndesaulniers@google.com>
To:     corbet@lwn.net
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
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Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
---
 Documentation/x86/exception-tables.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/Documentation/x86/exception-tables.rst b/Documentation/x86/exception-tables.rst
index ed6d4b0cf62c..15455b2f7ba8 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 +347,9 @@ 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.
-- 
2.25.1.696.g5e7596f4ac-goog