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

[Jan Beulich <jbeulich@xxxxxxxx>]

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:
>>> +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>> +
>>> +Xen can bundle microcode updates within its image. This support is 
>>> conditional
>>> +on the build configuration BUILTIN_UCODE being enabled. Builtin microcode 
>>> is
>>> +useful to ensure that, by default, a minimum microcode patch level will be
>>> +applied to the underlying CPU.
>>> +
>>> +To use microcode updates available on the build system as builtin,
>>> +use BUILTIN_UCODE_DIR to refer to the directory containing the firmware 
>>> updates
>>> +and specify the individual microcode patches via either BUILTIN_UCODE_AMD 
>>> or
>>> +BUILTIN_UCODE_INTEL for AMD microcode or INTEL microcode, respectively. For
>>> +instance, the configuration below is suitable for a build system which has 
>>> a
>>> +``/lib/firmware/`` directory which, in turn, includes the individual 
>>> microcode
>>> +patches ``amd-ucode/microcode_amd_fam15h.bin``, ``intel-ucode/06-3a-09``, 
>>> and
>>> +``intel-ucode/06-2f-02``.
>>> +
>>> +  CONFIG_BUILTIN_UCODE=y
>>> +  CONFIG_BUILTIN_UCODE_DIR="/lib/firmware/"
>>> +  CONFIG_BUILTIN_UCODE_AMD="amd-ucode/microcode_amd_fam15h.bin"
>>> +  CONFIG_BUILTIN_UCODE_INTEL="intel-ucode/06-3a-09 intel-ucode/06-2f-02"
>>
>> Rather than a blank as separator, the more conventional one on
>> Unix and alike would be : I think. Of course ideally there wouldn't
>> be any restriction at all on the characters usable here for file
>> names.
>>
> 
> It would be great if there is a particular convention. The blank 
> separator is aligned with Linux way of doing builtin microcode.

Well, this is then another area where I would question whether we
really want to follow the Linux approach, but I'm not bothered
enough to make less non-conventional behavior here a requirement.

>>> --- a/xen/arch/x86/Kconfig
>>> +++ b/xen/arch/x86/Kconfig
>>> @@ -218,6 +218,36 @@ config MEM_SHARING
>>>     bool "Xen memory sharing support" if EXPERT = "y"
>>>     depends on HVM
>>>   
>>> +config BUILTIN_UCODE
>>> +   bool "Support for Builtin Microcode"
>>> +   ---help---
>>> +     Include the CPU microcode update in the Xen image itself. With this
>>> +     support, Xen can update the CPU microcode upon boot using the builtin
>>> +     microcode, with no need for an additional microcode boot modules.
>>> +
>>> +     If unsure, say N.
>>
>> I continue to be unconvinced that this separate option is needed.
>> Albeit compared to the v1 approach I will agree that handling
>> would become more complicated without.
> 
> Any particular preference between the v1 vs v2 approach?

I definitely like the vendor separation.

>>> @@ -701,7 +747,13 @@ static int __init microcode_init(void)
>>>        */
>>>       if ( ucode_blob.size )
>>>       {
>>> +#ifdef CONFIG_BUILTIN_UCODE
>>> +        /* No need to destroy module mappings if builtin was used */
>>> +        if ( !ucode_builtin )
>>> +            bootstrap_map(NULL);
>>> +#else
>>>           bootstrap_map(NULL);
>>> +#endif
>>
>> First of all - is there no ucode unrelated side effect of this
>> invocation? I.e. can it safely be skipped?
> 
> Maybe I am missing something. Are you asking if we can safely skip the 
> bootstrap_map(NULL)? (Quoting your response on PATCH v2 2/4 "And of 
> course we really want these mappings to be gone")

Yes - my point is that invoking the function here may in
principle cover for other mappings. However - this is the
invocation you've added in an earlier patch, isn't it? In
which case omitting it should be fine. Nevertheless I don't
see and harm in invoking the function, i.e. I'd rather keep
the code here simple.

>>> --- /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?

>>> +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.

Jan

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