[PATCH RFC 3/3] Documentation: x86: microcode: add description for metadata file

[Mihai Carabas]

Microcode nowadays may remove of modify certain CPU feature bits. Prior
to this patch the kernel was blindly loading any microcode blob which might
have caused an unrecoverable error (e.g. the kernel was executing an
instruction that was removed). The following patches will process the
metadata file and will know what features are being added/removed/modified
and can take a decision on loading or not the new microcode blob.

Signed-off-by: Mihai Carabas <mihai.carabas@xxxxxxxxxx>
---
 Documentation/x86/microcode.rst | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/Documentation/x86/microcode.rst b/Documentation/x86/microcode.rst
index a320d37..45e3ae8 100644
--- a/Documentation/x86/microcode.rst
+++ b/Documentation/x86/microcode.rst
@@ -110,6 +110,42 @@ The loading mechanism looks for microcode blobs in
 /lib/firmware/{intel-ucode,amd-ucode}. The default distro installation
 packages already put them there.
 
+Late loading metadata file
+==========================
+
+New microcode blobs may remove or modify CPU feature bits. Prior to this
+metadata file, the microcode was blindly loaded and might have created an
+unrecoverable error (e.g. remove an instruction used currently in the kernel).
+
+In order to improve visibility on what features a new microcode that is being
+loaded at runtime (late loading) brings in, a new metadata file is created
+together with the microcode blob. The metadata file has the same name as the
+microcode blob with a suffix of ".metadata". The metadata file respects the
+following regular expression: "{m|c} {+|-} u32 [u32]*", where "m" means MSR
+feature and "c" means a CPUID exposed feature.
+
+Here is an example of content for the metadata file::
+   m + 0x00000122
+   m - 0x00000120
+   c + 0x00000007 0x00 0x00000000 0x021cbfbb 0x00000000 0x00000000
+   c - 0x00000007 0x00 0x00000000 0x021cbfbb 0x00000000 0x00000000
+
+The definition of the file format is as follows::
+   - each line contains an action on a CPU feature that the microcode will do
+   - the first letter specify the type of the feature
+   - the second letter specify the operation:
+   -- + - adds the feature
+   -- - - removes the feature
+   - the third letter specifies the index of the CPUID or the MSR
+   - for the CPUID case all the others parameters specifies the
+     leaf, eax, ebx, ecx and edx values
+
+Using this metadata file, the kernel, based on its internal policies, may
+deny a microcode update in order to ensure system stability (e.g. if an
+instruction is removed by the microcode and that instruction is still being
+used by the current code, we would drop the update as it would brake the
+system).
+
 Builtin microcode
 =================
 
-- 
1.8.3.1