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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.12.2019 23:40, Eslam Elnikety wrote:
> On 10.12.19 10:21, Jan Beulich wrote:
>> On 09.12.2019 22:49, Eslam Elnikety wrote:
>>> On 09.12.19 16:19, Andrew Cooper wrote:
>>>> On 09/12/2019 08:41, Eslam Elnikety wrote:
>>>>> --- /dev/null
>>>>> +++ b/xen/arch/x86/microcode/Makefile
>>>>> @@ -0,0 +1,40 @@
>>>>> +# 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.
>>>>> +
>>>>> +obj-y += builtin_ucode.o
>>>>> +
>>>>> +# Directory holding the microcode updates.
>>>>> +UCODE_DIR=$(patsubst "%",%,$(CONFIG_BUILTIN_UCODE_DIR))
>>>>> +amd-blobs := $(wildcard $(UCODE_DIR)/amd-ucode/*)
>>>>> +intel-blobs := $(wildcard $(UCODE_DIR)/intel-ucode/*)
>>>>
>>>> This is a little dangerous.  I can see why you want to do it like this,
>>>> and I can't provide any obvious suggestions, but if this glob picks up
>>>> anything which isn't a microcode file, it will break the logic to search
>>>> for the right blob.
>>>>
>>>
>>> We can limit the amd-blobs and intel-blob to binaries following the
>>> naming convention AuthenticAMD.bin and GenuineIntel.bin for amd and
>>> intel, respectively. Yet, this would be imposing an unnecessary
>>> restriction on administrators who may want to be innovative with naming
>>> (or want to use the naming microcode_amd_*.bin or FF-MM-SS instead).
>>>
>>> Alternatively, we can introduce CONFIG_BUILTIN_UCODE_INTEL and
>>> CONFIG_BUILTIN_UCODE_AMD. Both default to empty strings. Then, an
>>> administrator can specify exactly the microcodes to include relative to
>>> the CONFIG_BUILTIN_UCODE_DIR. For example:
>>> CONFIG_BUILTIN_UCODE_INTEL="intel-ucode/06-3a-09"
>>> CONFIG_BUILTIN_UCODE_AMD="amd-ucode/microcode_amd_fam15h.bin"
>>
>> This would make the feature even less generic - I already meant to
> 
> I do not follow the point about being less generic. (I hope my example 
> did not give the false impression that CONFIG_BUILTIN_UCODE_{AMD,INTEL} 
> allow for only a single microcode blob for a single signature).

Well, the example indeed has given this impression to me. I'm
having a hard time seeing how, beyond very narrow special cases,
either of the examples could be useful to anyone. Yet I think
examples should be generally useful.

>> ask whether building ucode into binaries is really a useful thing
>> when we already have more flexible ways. I could see this being
>> useful if there was no other way to make ucode available at boot
>> time.
> 
> It is useful in addition to the existing ways to do early microcode 
> updates. First, when operating many hosts, using boot modules (either a 
> distinct microcode module or within an initrd) becomes involved. For 
> instance, tools to update boot entries (e.g., 
> https://linux.die.net/man/8/grubby) do not support adding arbitrary 
> (microcode) modules.

I.e. you suggest to work around tools shortcomings by extending
Xen? Wouldn't the more appropriate way to deal with this be to
make the tools more capable?

> Second, there is often need to couple a Xen build with a minimum 
> microcode patch level. Having the microcode built within the Xen image 
> itself is a streamlined, natural way of achieving that.

Okay, I can accept this as a reason, to some degree at least. Yet
as said elsewhere, I don't think you want then to override a
possible "external" ucode module with the builtin blobs. Instead
the newest of everything that's available should then be loaded.

Jan

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