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Re: [Xen-devel] [PATCH v2 4/4] x86/microcode: Support builtin CPU microcode



On 20.12.19 11:12, Jan Beulich wrote:
On 19.12.2019 23:11, Eslam Elnikety wrote:
On 18.12.19 13:42, Jan Beulich wrote:
On 18.12.2019 02:32, Eslam Elnikety wrote:
--- /dev/null
+++ b/xen/arch/x86/microcode/Makefile
@@ -0,0 +1,46 @@
+# Copyright (C) 2019 Amazon.com, Inc. or its affiliates.
+# Author: Eslam Elnikety <elnikety@xxxxxxxxxx>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# Remove quotes and excess spaces from configuration strings
+UCODE_DIR=$(strip $(subst $\",,$(CONFIG_BUILTIN_UCODE_DIR)))
+UCODE_AMD=$(strip $(subst $\",,$(CONFIG_BUILTIN_UCODE_AMD)))
+UCODE_INTEL=$(strip $(subst $\",,$(CONFIG_BUILTIN_UCODE_INTEL)))
+
+# AMD and INTEL microcode blobs. Use 'wildcard' to filter for existing blobs.
+amd-blobs := $(wildcard $(addprefix $(UCODE_DIR),$(UCODE_AMD)))
+intel-blobs := $(wildcard $(addprefix $(UCODE_DIR),$(UCODE_INTEL)))
+
+ifneq ($(amd-blobs),)
+obj-y += ucode_amd.o
+endif
+
+ifneq ($(intel-blobs),)
+obj-y += ucode_intel.o
+endif
+
+ifeq ($(amd-blobs)$(intel-blobs),)
+obj-y += ucode_dummy.o
+endif
+
+ucode_amd.o: Makefile $(amd-blobs)
+       cat $(amd-blobs) > $@.bin
+       $(OBJCOPY) -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section 
.data=.builtin_amd_ucode,alloc,load,readonly,data,contents $@.bin $@
+       rm -f $@.bin
+
+ucode_intel.o: Makefile $(intel-blobs)
+       cat $(intel-blobs) > $@.bin
+       $(OBJCOPY) -I binary -O elf64-x86-64 -B i386:x86-64 --rename-section 
.data=.builtin_intel_ucode,alloc,load,readonly,data,contents $@.bin $@
+       rm -f $@.bin

This can be had with a pattern rule (with the vendor being the stem)
and hence without duplication, I think.

Also - is simply concatenating the blobs reliable enough? There's no
build time diagnostic that the result would actually be understood
at runtime.


Concatenation is reliable (as long as the individual microcode blobs are
not malformed, and in that case the builtin is not making matters worse
compared to presenting the malformed update via <integer> | scan).

A malformed update found the other way is a bug in the tools
constructing the respective images. A malformed built-in
update is a bug in the Xen build system. The put the question
differently: Is it specified somewhere that the blobs all have
to have certain properties, which the straight concatenation
relies upon?


Refer to ( https://www.kernel.org/doc/Documentation/x86/microcode.txt ). AuthenticAMD.bin and GenuineIntel.bin are both concatenations of individual microcode blobs.

+ucode_dummy.o: Makefile
+       $(CC) $(CFLAGS) -c -x c /dev/null -o $@;

Since the commit message doesn't explain why this is needed, I
have to ask (I guess we somewhere have a dependency on $(obj-y)
not being empty).

Your guess is correct. All sub-directories of xen/arch/x86 are expected
to produce built_in.o. If there are not amd nor intel microcode blobs,
there will be no build dependencies and the build fails preparing the
built_in.o

That's rather poor, but it's of course not your task to get this
fixed (it shouldn't be very difficult to create an empty
built_in.o for an empty $(obj-y)).

_If_ it is needed, I don't see why you need
ifeq() around its use. In fact you could have

obj-y := ucode-dummy.o

right at the top of the file.

Furthermore I don't really understand why you need this in the
first place. While cat won't do what you want with an empty
argument list, can't you simply prepend / append /dev/null?


To make sure we are on the same page. You are suggesting using
"/dev/null" in case there are no amd/intel ucode to generate the
ucode_amd/intel.o? If so, objcopy does not allow using /dev/null as
input (complains about empty binary).

That's again rather poor, this time of the utility - it should be
easy enough to produce an object with an empty .data (or whatever
it is) section. As above - I'm fine with you keeping the logic
then as is, provided you say in the description why it can't be
simplified.

Ack. Will justify the logic in comments.

-- Eslam


Jan



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