linux-kernel - Re: [PATCH v3] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode

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Date:   Sun, 5 Jul 2020 18:23:04 +0300
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Doug Anderson <dianders@...gle.com>
Cc:     Abhishek Bhardwaj <abhishekbh@...gle.com>,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Waiman Long <longman@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "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>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tony Luck <tony.luck@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>, kvm@...r.kernel.org,
        x86 <x86@...nel.org>
Subject: Re: [PATCH v3] x86/speculation/l1tf: Add KConfig for setting the L1D
 cache flush mode

On Fri, Jul 03, 2020 at 07:00:11AM -0700, Doug Anderson wrote:
> Hi,
> 
> On Fri, Jul 3, 2020 at 4:40 AM Mike Rapoport <rppt@...ux.ibm.com> wrote:
> >
> > On Thu, Jul 02, 2020 at 11:43:47PM -0700, Abhishek Bhardwaj wrote:
> > > We have tried to steer away from kernel command line args for a few reasons.
> > >
> > > I am paraphrasing my colleague Doug's argument here (CC'ed him as well) -
> > >
> > > - The command line args are getting unwieldy. Kernel command line
> > > parameters are not a scalable way to set kernel config. It's 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 kernel behavior.
> >
> > Why cannot you simply add this option to CONFIG_CMDLINE at your kernel build
> > scripts?
> 
> At least in the past I've seen that 'CONFIG_CMDLINE' interacts badly
> with the bootloader provided command line in some architectures.  In
> days of yore I tried to post a patch to fix this, at least on ARM
> targets, but it never seemed to go anywhere upstream.  I'm going to
> assume this is still a problem because I still see an ANDROID tagged
> patch in the Chrome OS 5.4 tree:

I presume a patch subject should have been here :)
Anyway, bad iteraction of CONFIG_CMDLINE with bootloader command line
seems like a bug to me and a bug need to be fixed.

> In any case, as per my previous arguments, stuffing lots of config
> into the cmdline is a bit clunky and doesn't scale well.  You end up
> with a really long run on command line and it's hard to tell where one
> config option ends and the next one starts and if the same concept is
> there more than one time it's hard to tell and something might cancel
> out a previous config option or maybe it won't and by the time you end
> up finishing this it's hard to tell where you started.  :-)

Configuration options may also have weird interactions between them and
addition of #ifdef means that most of the non-default paths won't get as
good test coverage as the default one.

And the proposed #ifdef maze does not look pretty at all...

> > > - Also, we know we want this setting from the start. This is a
> > > definite smell that it deserves to be a compile time thing rather than
> > > adding extra code + whatever miniscule time at runtime to pass an
> > > extra arg.
> >
> > This might be a compile time thing in your environment, but not
> > necessarily it must be the same in others. For instance, what option
> > should distro kernels select?
> 
> Nothing prevents people from continuing to use the command line
> options if they want, right?  This just allows a different default.
> So if a distro is security focused and decided that it wanted a slower
> / more secure default then it could ship that way but individual users
> could still override, right?

Well, nothing prevents you from continuing to use the command line as
well ;-)

I can see why whould you want an ability to select compile time default
for an option, but I'm really not thrilled by the added ifdefery.
 
> > > I think this was what CONFIGS were intended for. I'm happy to add all
> > > this to the commit message once it's approved in spirit by the
> > > maintainers.
> > >

-- 
Sincerely yours,
Mike.