RE: [PATCH v2 03/35] x86/bugs: Add AUTO mitigations for mds/taa/mmio/rfds

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> Sent: Thursday, November 14, 2024 11:14 AM
> To: Kaplan, David <David.Kaplan@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Peter
> Zijlstra <peterz@xxxxxxxxxxxxx>; Josh Poimboeuf <jpoimboe@xxxxxxxxxx>; Ingo
> Molnar <mingo@xxxxxxxxxx>; Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>;
> x86@xxxxxxxxxx; H . Peter Anvin <hpa@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2 03/35] x86/bugs: Add AUTO mitigations for
> mds/taa/mmio/rfds
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Thu, Nov 14, 2024 at 02:59:34PM +0000, Kaplan, David wrote:
> > [AMD Official Use Only - AMD Internal Distribution Only]
> >
> > > -----Original Message-----
> > > From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> > > Sent: Wednesday, November 13, 2024 8:27 PM
> > > To: Kaplan, David <David.Kaplan@xxxxxxx>
> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Borislav Petkov
> > > <bp@xxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Josh
> > > Poimboeuf <jpoimboe@xxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>;
> > > Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx; H . Peter
> > > Anvin <hpa@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> > > Subject: Re: [PATCH v2 03/35] x86/bugs: Add AUTO mitigations for
> > > mds/taa/mmio/rfds
> > >
> > > Caution: This message originated from an External Source. Use proper
> > > caution when opening attachments, clicking links, or responding.
> > >
> > >
> > > On Tue, Nov 05, 2024 at 03:54:23PM -0600, David Kaplan wrote:
> > > > @@ -1995,6 +2004,7 @@ void cpu_bugs_smt_update(void)
> > > >               update_mds_branch_idle();
> > > >               break;
> > > >       case MDS_MITIGATION_OFF:
> > > > +     case MDS_MITIGATION_AUTO:
> > >
> > > This implies AUTO and OFF are similar, which is counter intuitive.
> > > While mitigation selection code ...
> > >
> > > > +     if (mds_mitigation == MDS_MITIGATION_AUTO)
> > > > +             mds_mitigation = MDS_MITIGATION_FULL;
> > > > +
> > >
> > > ... indicates that AUTO is equivalent to FULL. So, I think AUTO
> > > should be handled the same way as FULL in cpu_bugs_smt_update() as well.
> > >
> > > Same for TAA and MMIO below.
> > >
> >
> > The mitigation is never actually AUTO by the time we call
> > cpu_bugs_smt_update(), since this happens after cpu_select_mitigations().
> > I had to add the case statement here so the switch statement was
> > complete, but this case will never be hit.
> >
> > Should I put a comment here about that?  Or is a default case the
> > better way to handle this?
>
> My suggestion would be to treat AUTO as FULL, and move it up with FULL:
>
>          switch (mds_mitigation) {
>          case MDS_MITIGATION_FULL:
> +        case MDS_MITIGATION_AUTO:
>          case MDS_MITIGATION_VMWERV:
>                  if (sched_smt_active() && !boot_cpu_has(X86_BUG_MSBDS_ONLY))
>                          pr_warn_once(MDS_MSG_SMT);
>                  update_mds_branch_idle();
>                  break;
>          case MDS_MITIGATION_OFF:
>                  break;
>          }

Ok, I can do that

Thanks --David Kaplan