Re: [PATCH v2] x86/ucode: Adjust parse_ucode() to match other list handling

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.12.2025 16:32, Andrew Cooper wrote:
> parse_ucode() is abnormal compared to similar parsing elsewhere in Xen.
> 
> Invert the ucode_mod_forced check with respect to the "scan" and integer
> handling, so we can warn the user when we've elected to ignore the parameters,
> and yield -EINVAL for any unrecognised list element.
> 
> Rewrite the ucode= command line docs for clarity.
> 
> No practical change.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I'm not quite happy with ...

> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -2752,34 +2752,52 @@ performance.
>     Alternatively, selecting `tsx=1` will re-enable TSX at the users own risk.
>  
>  ### ucode
> -> `= List of [ <integer> | scan=<bool>, nmi=<bool>, digest-check=<bool> ]`
> +> `= List of [ <integer>, scan=<bool>, nmi=<bool>, digest-check=<bool> ]`

... this change when ...

>      Applicability: x86
>      Default: `scan` is selectable via Kconfig, `nmi,digest-check`
>  
> -Controls for CPU microcode loading. For early loading, this parameter can
> -specify how and where to find the microcode update blob. For late loading,
> -this parameter specifies if the update happens within a NMI handler.
> -
> -'integer' specifies the CPU microcode update blob module index. When 
> positive,
> -this specifies the n-th module (in the GrUB entry, zero based) to be used
> -for updating CPU micrcode. When negative, counting starts at the end of
> -the modules in the GrUB entry (so with the blob commonly being last,
> -one could specify `ucode=-1`). Note that the value of zero is not valid
> -here (entry zero, i.e. the first module, is always the Dom0 kernel
> -image). Note further that use of this option has an unspecified effect
> -when used with xen.efi (there the concept of modules doesn't exist, and
> -the blob gets specified via the `ucode=<filename>` config file/section
> -entry; see [EFI configuration file description](efi.html)).
> -
> -'scan' instructs the hypervisor to scan the multiboot images for an cpio
> -image that contains microcode. Depending on the platform the blob with the
> -microcode in the cpio name space must be:
> -  - on Intel: kernel/x86/microcode/GenuineIntel.bin
> -  - on AMD  : kernel/x86/microcode/AuthenticAMD.bin
> -When using xen.efi, the `ucode=<filename>` config file setting takes
> -precedence over `scan`. The default value for `scan` is set with
> -`CONFIG_UCODE_SCAN_DEFAULT`.
> +Controls for CPU microcode loading.
> +
> +In order to load microcode at boot, Xen needs to find a suitable update
> +amongst the modules provided by the bootloader.  Two kinds of microcode 
> update
> +are supported:
> +
> + 1. Raw microcode containers.  The format of the container is CPU vendor
> +    specific.
> +
> + 2. CPIO archive.  This is Linux's preferred mechanism, and involves having
> +    the raw containers expressed as files
> +    (e.g. `kernel/x86/microcode/{GenuineIntel,AuthenticAMD}.bin`) in a CPIO
> +    archive, typically prepended to the initrd.
> +
> +The `<integer>` and `scan=<bool>` options are mutually exclusive and select
> +between these two options.  Further restrictions exist for booting xen.efi
> +(see below).

... then you say verbally that the two are exclusive of one another. That's
what the | there was intended to indicate. IOW I would prefer that line to
be left alone, but I'm not intending to insist.

Jan