Re: [PATCH 4/5] x86/ucode: Cache results in microcode_scan_module()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.03.2023 18:07, Andrew Cooper wrote:
> On 28/03/2023 5:01 pm, Jan Beulich wrote:
>> On 28.03.2023 17:12, Andrew Cooper wrote:
>>> On 28/03/2023 3:19 pm, Jan Beulich wrote:
>>>> On 27.03.2023 21:41, Andrew Cooper wrote:
>>>>> Right now, the boot flow depends on the second pass over
>>>>> bootstrap_map()/find_cpio_data() altering ucode_blob.data to use the 
>>>>> directmap
>>>>> alias of the CPIO module, where previously it caches the early boostrap
>>>>> mapping.
>>>>>
>>>>> If the scan is successful, it will be successful the second time too, but
>>>>> there's no point repeating the work.  Cache the module index, offset and 
>>>>> size
>>>>> to short circuit things the second time around.
>>>> If the scan failed, it will fail the 2nd time too. Maybe deal with
>>>> this case as well, e.g. by clearing ucode_scan at the end of
>>>> microcode_scan_module() when nothing was found?
>>> See patch 5.  It can only become true then because of how the callers
>>> are arranged.
>> Right, I've meanwhile seen you do it there. That's fine. Yet I think it
>> could also be done earlier (and if I'm not mistaken also ahead of all
>> of the rearrangements you do).
> 
> Prior to patch 5, calls into scan are guarded with "if ( ucode_scan )"
> as well as there being an "if ( !ucode_scan )" check.
> 
> Clobbering ucode_scan prior to patch 5 breaks the second pass to cache
> the ucode we loaded on the BSP.

How that? We didn't load anything if scan failed on the first pass, and
clearing ucode_scan if the first pass failed would merely suppress a
fruitless second pass. Of course we cannot clear ucode_scan in the case
that the first pass succeeded, but my earlier suggestion was limited to
the failure case.

The "!ucode_scan" check also is dead code, btw (and hence doesn't matter
here), since neither caller would invoke the function in that case in
the first place.

Jan