linux-kernel - Re: [PATCH 7/7] x86/bugs: Replace CONFIG_SPECTRE_BHI_{ON,OFF} with CONFIG_MITIGATION_SPECTRE

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Date: Thu, 11 Apr 2024 10:18:16 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexandre Chartre <alexandre.chartre@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sean Christopherson <seanjc@...gle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Nikolay Borisov <nik.borisov@...e.com>,
	KP Singh <kpsingh@...nel.org>, Waiman Long <longman@...hat.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 7/7] x86/bugs: Replace CONFIG_SPECTRE_BHI_{ON,OFF} with
 CONFIG_MITIGATION_SPECTRE_BHI

* Ingo Molnar <mingo@...nel.org> wrote:

> >  static enum bhi_mitigations bhi_mitigation __ro_after_init =
> > -	IS_ENABLED(CONFIG_SPECTRE_BHI_ON) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
> > +	IS_ENABLED(CONFIG_MITIGATION_SPECTRE_BHI_ON) ? BHI_MITIGATION_ON : BHI_MITIGATION_OFF;
> 
> Uhm, after this patch there's no CONFIG_MITIGATION_SPECTRE_BHI_ON 
> anymore, which is kindof nasty, as IS_ENABLED() doesn't generate a build 
> failure for non-existent Kconfig variables IIRC ...
> 
> So AFAICT this patch turns on BHI unconditionally.

BTW., this is why IS_ENABLED() is a bad primitive IMO:

kepler:~/tip> for N in $(git grep -w IS_ENABLED arch/x86/ | sed 's/.*IS_ENABLED(//g' | sed 's/).*//g' | sort | uniq | sed 's/^CONFIG_//g'); do printf "# %40s: " $N; git grep -E "^config $N$" -- '**Kconfig**' | wc -l; done | grep -w 0
#           RESCTRL_RMID_DEPENDS_ON_CLOSID: 0
#                   NODE_NOT_IN_PAGE_FLAGS: 0

1)

CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID doesn't exist anymore, but is used 
widely:

 kepler:~/tip> git grep RESCTRL_RMID_DEPENDS_ON_CLOSID
 arch/x86/kernel/cpu/resctrl/monitor.c: *     Only allocated when CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID is defined.
 arch/x86/kernel/cpu/resctrl/monitor.c:  if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
 arch/x86/kernel/cpu/resctrl/monitor.c:  if (!IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
 arch/x86/kernel/cpu/resctrl/monitor.c:  if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID))
 arch/x86/kernel/cpu/resctrl/monitor.c:  if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID)) {
 arch/x86/kernel/cpu/resctrl/monitor.c:          if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID)) {
 arch/x86/kernel/cpu/resctrl/monitor.c:  if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID)) {
 arch/x86/kernel/cpu/resctrl/rdtgroup.c: if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID)) {

Each of those uses is bogus, as the Kconfig symbol doesn't exist. (!)

AFAICT CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID was never defined within the 
upstream kernel (!!).

AFAICT the first bogus CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID use was 
introduced in this recent commit:

   b30a55df60c3 ("x86/resctrl: Track the number of dirty RMID a CLOSID has")

.. and was cargo-cult copied in other patches. It was never explained in 
the changelog why it's used without a Kconfig entry anywhere.

Maybe in the future some other arch might (or might not) introduce 
RESCTRL_RMID_DEPENDS_ON_CLOSID, but that doesn't justify this bad pattern 
of dead code ...

2)

The NODE_NOT_IN_PAGE_FLAGS use is borderline correct:

  kepler:~/tip> git grep -w NODE_NOT_IN_PAGE_FLAGS arch/x86
  arch/x86/mm/numa.c:     if (IS_ENABLED(NODE_NOT_IN_PAGE_FLAGS)) {

As NODE_NOT_IN_PAGE_FLAGS is not a Kconfig symbol, but a flag defined by 
the VM:

  include/linux/page-flags-layout.h:#define NODE_NOT_IN_PAGE_FLAGS        1

.. which happens to work with how IS_ENABLED() is defined. No other user 
of NODE_NOT_IN_PAGE_FLAGS uses the IS_ENABLED() pattern.

Thanks,

	Ingo