Re: [PATCH] x86/ucode: Add option to scan microcode by default

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On Mon, Feb 17, 2025 at 4:44 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 17/02/2025 4:08 pm, Ross Lagerwall wrote:
> > A lot of systems automatically add microcode to the initrd so it can be
> > useful as a vendor policy to always scan for microcode. Add a Kconfig
> > option to allow setting the default behaviour.
> >
> > The default behaviour is unchanged since the new option defaults to
> > "no".
> >
> > Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>
> I'm pretty sure this is safe on 4.20.
>
> Prior versions of Xen need the fix to make idx/scan be mutually
> exclusive, which hasn't been backported yet IIRC.
>
> > ---
> >  xen/arch/x86/Kconfig              | 11 +++++++++++
> >  xen/arch/x86/cpu/microcode/core.c |  2 +-
> >  2 files changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
> > index 9cdd04721afa..b806d8bc3319 100644
> > --- a/xen/arch/x86/Kconfig
> > +++ b/xen/arch/x86/Kconfig
> > @@ -383,6 +383,17 @@ config ALTP2M
> >
> >         If unsure, stay with defaults.
> >
> > +config UCODE_SCAN_DEFAULT
> > +     def_bool n
> > +     prompt "Scan for microcode by default"
> > +     help
> > +       During boot, Xen can scan the multiboot images for a CPIO archive
> > +       containing CPU microcode to be loaded.
>
> ", which is Linux's mechanism for early microcode loading."
>
> This is quite an important point to cover.
>
> > +
> > +       Enabling this option will cause Xen to scan for it by default.
> > +
> > +       If unsure, say N.
>
> Personally I don't like this "If unsure", because it's almost always
> redundant with the default.
>
> In this case, it really ought to be "set if you have a Linux dom0",
> which in turn begs the question as to whether it ought to be bool y.
>

I agree with you that changing the default to yes is probably a good
idea but I think it would be better to separate making it configurable
from changing the default behaviour.

Ross