linux-kernel - Re: [PATCH 1/3] x86/bugs: Create an option to disable MDS

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Message-ID: <ZIxLt3/vWyoB/UIK@gmail.com>
Date:   Fri, 16 Jun 2023 04:47:03 -0700
From:   Breno Leitao <leitao@...ian.org>
To:     Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc:     jpoimboe@...nel.org, peterz@...radead.org, tglx@...utronix.de,
        bp@...en8.de, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, leit@...com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] x86/bugs: Create an option to disable MDS

On Thu, Jun 15, 2023 at 03:13:47PM -0700, Pawan Gupta wrote:
> On Thu, Jun 15, 2023 at 09:44:12AM -0700, Breno Leitao wrote:
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 182af64387d0..50f12829dce9 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -269,6 +269,7 @@ static void x86_amd_ssb_disable(void)
> >  /* Default mitigation for MDS-affected CPUs */
> >  static enum mds_mitigations mds_mitigation __ro_after_init = MDS_MITIGATION_FULL;
> >  static bool mds_nosmt __ro_after_init = false;
> > +#define MDS_WARN_MSG "WARNING: Microarchitectural Data Sampling (MDS) speculative mitigation disabled!\n"
> >  
> >  static const char * const mds_strings[] = {
> >  	[MDS_MITIGATION_OFF]	= "Vulnerable",
> > @@ -278,11 +279,17 @@ static const char * const mds_strings[] = {
> >  
> >  static void __init mds_select_mitigation(void)
> >  {
> > -	if (!boot_cpu_has_bug(X86_BUG_MDS) || cpu_mitigations_off()) {
> > +	if (!boot_cpu_has_bug(X86_BUG_MDS)) {
> >  		mds_mitigation = MDS_MITIGATION_OFF;
> >  		return;
> >  	}
> >  
> > +	if (cpu_mitigations_off() || !IS_ENABLED(CONFIG_MITIGATE_MDS)) {
> > +		mds_mitigation = MDS_MITIGATION_OFF;
> > +		pr_err(MDS_WARN_MSG);
> > +		return;
> 
> Why does compile time config needs to be so restrictive that it does not
> allow runtime override with mds= cmdline?
> 
> I believe Kconfig should only be setting the mitigation default,
> allowing users to select mitigation at runtime:

Sure, that is doable as well. If no one has any opposition to it, I will
implemented as suggested.