linux-kernel - Re: [PATCH v5] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode

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Date:   Sun, 16 Aug 2020 09:47:25 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Abhishek Bhardwaj <abhishekbh@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Mark Gross <mgross@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Waiman Long <longman@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>, kvm@...r.kernel.org,
        x86@...nel.org, Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v5] x86/speculation/l1tf: Add KConfig for setting the L1D
 cache flush mode

+ Masami.

On Thu, Jul 09, 2020 at 12:51:34PM +0200, Thomas Gleixner wrote:
> Abhishek Bhardwaj <abhishekbh@...gle.com> writes:
> > This change adds a new kernel configuration that sets the l1d cache
> > flush setting at compile time rather than at run time.
> >
> > The reasons for this change are as follows -
> >
> >  - Kernel command line arguments are getting unwieldy. These parameters
> >  are not a scalable way to set the kernel config. They're intended as a
> >  super limited way for the bootloader to pass info to the kernel and
> >  also as a way for end users who are not compiling the kernel themselves
> >  to tweak the kernel behavior.
> >
> >  - Also, if a user wants this setting from the start. It's a definite
> >  smell that it deserves to be a compile time thing rather than adding
> >  extra code plus whatever miniscule time at runtime to pass an
> >  extra argument.
> >
> >  - Finally, it doesn't preclude the runtime / kernel command line way.
> >  Users are free to use those as well.
> 
> TBH, I don't see why this is a good idea.
> 
>  1) I'm not following your argumentation that the command line option is
>     a poor Kconfig replacement. The L1TF mode is a boot time (module
>     load time) decision and the command line parameter is there to
>     override the carefully chosen and sensible default behaviour.
> 
>  2) You can add the desired mode to the compiled in (partial) kernel
>     command line today.
> 
>  3) Boot loaders are well capable of handling large kernel command lines
>     and the extra time spend for reading the parameter does not matter
>     at all.

Also, there's Documentation/admin-guide/bootconfig.rst which extends
cmdline options handling even more and allows for passing options in a
file. Maybe that'll help in this case too.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette