Re: [PATCH 2/2] x86/ucode: Drop the ucode=nmi option

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.02.2025 23:29, Andrew Cooper wrote:
> This option is active by default, and despite what the documentation suggests
> about choosing ucode=no-nmi, it only controls whether the primary threads move
> into NMI context.
> 
> Sibling threads unconditionally move into NMI context, which is confirmed by
> an in-code comment.
> 
> Not discussed is the fact that the BSP never enters NMI context, which works
> only by luck (AMD CPUs, where we reload on sibling threads too, has working
> in-core rendezvous and doesn't require NMI cover on the primary thread for
> safety).  This does want addressing, but requires more untangling first.
> 
> Overall, `ucode=no-nmi` is a misleading and pretty useless option.  Drop it,
> and simplify the two users.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Despite the reasonably large diff in primary_thread_fn(), it's mostly just
> unindenting the block, and dropping the final call to primary_thread_work()
> which is made dead by this change.
> ---
>  docs/misc/xen-command-line.pandoc |  8 ++-----
>  xen/arch/x86/cpu/microcode/core.c | 38 +++++++++++--------------------
>  2 files changed, 15 insertions(+), 31 deletions(-)
> 
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index 47674025249a..9702c36b8c39 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -2721,10 +2721,10 @@ performance.
>     Alternatively, selecting `tsx=1` will re-enable TSX at the users own risk.
>  
>  ### ucode
> -> `= List of [ <integer>, scan=<bool>, nmi=<bool> ]`
> +> `= List of [ <integer>, scan=<bool ]`

With this (taking my comment on patch 1 into account as well) I think ...

> @@ -123,9 +120,7 @@ static int __init cf_check parse_ucode(const char *s)
>          if ( !ss )
>              ss = strchr(s, '\0');
>  
> -        if ( (val = parse_boolean("nmi", s, ss)) >= 0 )
> -            ucode_in_nmi = val;
> -        else if ( (val = parse_boolean("scan", s, ss)) >= 0 )
> +        if ( (val = parse_boolean("scan", s, ss)) >= 0 )
>          {
>              if ( ucode_mod_forced )
>                  printk(XENLOG_WARNING

... this function will want to transition back to what it was prior to
the addition of the sub-option, preferably adjusted to account for the
possibility of multiple "ucode=" on the command line, i.e. along the
lines of

    if ( !strncmp(s, "scan", 4) )
    {
        ucode_scan = 1;
        ucode_mod_idx = 0;
    }
    else
    {
        ucode_scan = 0;
        ucode_mod_idx = simple_strtol(s, &q, 0);
    }

That would then make patch 1 kind of unnecessary.
    
Jan