Re: [PATCH v2] x86/mce: work around an erratum on fast string copy instructions.

[Jue Wang <juew@google.com>]

On Fri, Feb 18, 2022 at 7:07 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Thu, Feb 17, 2022 at 05:32:09PM -0800, Jue Wang wrote:
> > +static noinstr bool quirk_skylake_repmov(void)
> > +{
> > +     u64 mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
> > +     u64 misc_enable = __rdmsr(MSR_IA32_MISC_ENABLE);
> > +
> > +     /*
> > +      * Apply the quirk only to local machine checks, i.e., no broadcast
> > +      * sync is needed.
> > +      */
> > +     if ((mcgstatus & MCG_STATUS_LMCES) &&
> > +         unlikely(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
> > +             u64 mc1_status = mce_rdmsrl(MSR_IA32_MCx_STATUS(1));
> > +
> > +             /* Check for a software-recoverable data fetch error. */
> > +             if ((mc1_status &
> > +                  (MCI_STATUS_VAL | MCI_STATUS_OVER | MCI_STATUS_UC | MCI_STATUS_EN |
> > +                   MCI_STATUS_ADDRV | MCI_STATUS_MISCV | MCI_STATUS_PCC |
> > +                   MCI_STATUS_AR | MCI_STATUS_S)) ==
> > +                  (MCI_STATUS_VAL |                   MCI_STATUS_UC | MCI_STATUS_EN |
> > +                   MCI_STATUS_ADDRV | MCI_STATUS_MISCV |
> > +                   MCI_STATUS_AR | MCI_STATUS_S)) {
> > +                     misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
> > +                     __wrmsr(MSR_IA32_MISC_ENABLE,
> > +                             (u32)misc_enable, (u32)(misc_enable >> 32));
>
> "You're going to have to use the mce_{rd,wr}msrl() routines."
>
> I actually really meant that.
Thanks.

Since MSR_IA32_MISC_ENABLE is not a MCA register, I wonder if we want
to mix its read/write with the injected MCE code. I was a bit concerned about
potential race with mce-inject and the read/write to MSR_IA32_MISC_ENABLE.

>
> And I went and simplified this a bit more so that it is more readable,
> diff ontop.
>
> Also, Tony, I think the clearing of MCG_STATUS should happen last.
>
> ---
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index c1a41da99975..1741be9b9464 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -831,34 +831,35 @@ quirk_sandybridge_ifu(int bank, struct mce *m, struct pt_regs *regs)
>   */
>  static noinstr bool quirk_skylake_repmov(void)
>  {
> -       u64 mcgstatus = mce_rdmsrl(MSR_IA32_MCG_STATUS);
> -       u64 misc_enable = __rdmsr(MSR_IA32_MISC_ENABLE);
> +       u64 mcgstatus   = mce_rdmsrl(MSR_IA32_MCG_STATUS);
> +       u64 misc_enable = mce_rdmsrl(MSR_IA32_MISC_ENABLE);
> +       u64 mc1_status;
>
>         /*
>          * Apply the quirk only to local machine checks, i.e., no broadcast
>          * sync is needed.
>          */
> -       if ((mcgstatus & MCG_STATUS_LMCES) &&
> -           (misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING)) {
> -               u64 mc1_status = mce_rdmsrl(MSR_IA32_MCx_STATUS(1));
> -
> -               /* Check for a software-recoverable data fetch error. */
> -               if ((mc1_status &
> -                    (MCI_STATUS_VAL | MCI_STATUS_OVER | MCI_STATUS_UC | MCI_STATUS_EN |
> -                     MCI_STATUS_ADDRV | MCI_STATUS_MISCV | MCI_STATUS_PCC |
> -                     MCI_STATUS_AR | MCI_STATUS_S)) ==
> -                    (MCI_STATUS_VAL |                   MCI_STATUS_UC | MCI_STATUS_EN |
> -                     MCI_STATUS_ADDRV | MCI_STATUS_MISCV |
> -                     MCI_STATUS_AR | MCI_STATUS_S)) {
> -                       misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
> -                       __wrmsr(MSR_IA32_MISC_ENABLE,
> -                               (u32)misc_enable, (u32)(misc_enable >> 32));
> -                       mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
> -                       mce_wrmsrl(MSR_IA32_MCx_STATUS(1), 0);
> -                       pr_err_once("Errata detected, disable fast string copy instructions.\n");
> -                       return true;
> -               }
> +       if (!(mcgstatus & MCG_STATUS_LMCES) ||
> +           !(misc_enable & MSR_IA32_MISC_ENABLE_FAST_STRING))
> +               return false;
> +
> +       mc1_status = mce_rdmsrl(MSR_IA32_MCx_STATUS(1));
> +
> +       /* Check for a software-recoverable data fetch error. */
> +       if ((mc1_status &
> +            (MCI_STATUS_VAL | MCI_STATUS_OVER | MCI_STATUS_UC | MCI_STATUS_EN |
> +             MCI_STATUS_ADDRV | MCI_STATUS_MISCV | MCI_STATUS_PCC |
> +             MCI_STATUS_AR | MCI_STATUS_S)) ==
> +            (MCI_STATUS_VAL |                   MCI_STATUS_UC | MCI_STATUS_EN |
> +             MCI_STATUS_ADDRV | MCI_STATUS_MISCV |
> +             MCI_STATUS_AR | MCI_STATUS_S)) {
> +               misc_enable &= ~MSR_IA32_MISC_ENABLE_FAST_STRING;
> +               mce_wrmsrl(MSR_IA32_MISC_ENABLE, misc_enable);
> +               mce_wrmsrl(MSR_IA32_MCx_STATUS(1), 0);
> +               pr_err_once("Erratum detected, disable fast string copy instructions.\n");
> +               return true;
>         }
> +
>         return false;
>  }
>
> @@ -1432,7 +1433,7 @@ noinstr void do_machine_check(struct pt_regs *regs)
>                 return unexpected_machine_check(regs);
>
>         if (mce_flags.skx_repmov_quirk && quirk_skylake_repmov())
> -               return;
> +               goto clear;
>
>         /*
>          * Establish sequential order between the CPUs entering the machine
> @@ -1576,6 +1577,7 @@ noinstr void do_machine_check(struct pt_regs *regs)
>  out:
>         instrumentation_end();
>
> +clear:
>         mce_wrmsrl(MSR_IA32_MCG_STATUS, 0);
>  }
>  EXPORT_SYMBOL_GPL(do_machine_check);
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette