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

From: Mike Rapoport <rppt@linux.ibm.com>
To: Abhishek Bhardwaj <abhishekbh@google.com>
Cc: Anthony Steinhauser <asteinhauser@google.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Waiman Long <longman@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Mark Gross <mgross@linux.intel.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Tony Luck <tony.luck@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	kvm@vger.kernel.org, x86 <x86@kernel.org>,
	Doug Anderson <dianders@google.com>
Subject: Re: [PATCH v3] x86/speculation/l1tf: Add KConfig for setting the L1D cache flush mode
Date: Fri, 3 Jul 2020 14:40:37 +0300	[thread overview]
Message-ID: <20200703114037.GD2999146@linux.ibm.com> (raw)
In-Reply-To: <CA+noqoj6u9n_KKohZw+QCpD-Qj0EgoCXaPEsryD7ABZ7QpqQfg@mail.gmail.com>

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?

> - 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?

> 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.
> 
> On Thu, Jul 2, 2020 at 8:18 PM Anthony Steinhauser
> <asteinhauser@google.com> wrote:
> >
> > Yes, this probably requires an explanation why the change is necessary
> > or useful. Without that it is difficult to give some meaningful
> > feedback.
> 
> 
> 
> -- 
> Abhishek

-- 
Sincerely yours,
Mike.