Re: [PATCH] x86/tsx: Cope with RTM_ALWAYS_ABORT vs RTM mismatch

[Jan Beulich <jbeulich@suse.com>]

On 04.04.2024 12:41, Andrew Cooper wrote:
> @@ -9,6 +10,7 @@
>   *  -1 => Default, altered to 0/1 (if unspecified) by:
>   *                 - TAA heuristics/settings for speculative safety
>   *                 - "TSX vs PCR3" select for TSX memory ordering safety
> + *  -2 => Implicit tsx=0 (from RTM_ALWAYS_ABORT vs RTM mismatch)
>   *  -3 => Implicit tsx=1 (feed-through from spec-ctrl=0)
>   *
>   * This is arranged such that the bottom bit encodes whether TSX is actually
> @@ -114,11 +116,50 @@ void tsx_init(void)
>  
>          if ( cpu_has_tsx_force_abort )
>          {
> +            uint64_t val;
> +
>              /*
> -             * On an early TSX-enable Skylake part subject to the memory
> +             * On an early TSX-enabled Skylake part subject to the memory
>               * ordering erratum, with at least the March 2019 microcode.
>               */
>  
> +            rdmsrl(MSR_TSX_FORCE_ABORT, val);
> +
> +            /*
> +             * At the time of writing (April 2024), it was discovered that
> +             * some parts (e.g. CoffeeLake 8th Gen, 06-9e-0a, ucode 0xf6)
> +             * advertise RTM_ALWAYS_ABORT, but XBEGIN instructions #UD.  Other
> +             * similar parts (e.g. KabyLake Xeon-E3, 06-9e-09, ucode 0xf8)
> +             * operate as expected.
> +             *
> +             * In this case:
> +             *  - RTM_ALWAYS_ABORT and MSR_TSX_FORCE_ABORT are enumerated.
> +             *  - XBEGIN instructions genuinely #UD.
> +             *  - MSR_TSX_FORCE_ABORT is write-discard and fails to hold its
> +             *    value.
> +             *  - HLE and RTM are not enumerated, despite
> +             *    MSR_TSX_FORCE_ABORT.TSX_CPUID_CLEAR being clear.

Of these 4 items you use the first and last here. It took me some time to
figure that the middle two are (aiui) only informational, and that you
assume that first and last together are sufficient to uniquely identify
the problematic parts. Separating the two groups a little might be helpful.

For the write-discard property, how was that determined? Does it affect all
writable bits?

> +             * Spot this case, and treat it as if no TSX is available at all.
> +             * This will prevent Xen from thinking it's safe to offer HLE/RTM
> +             * to VMs.
> +             */
> +            if ( val == 0 && cpu_has_rtm_always_abort && !cpu_has_rtm )
> +            {
> +                printk(XENLOG_ERR
> +                       "FIRMWARE BUG: CPU %02x-%02x-%02x, ucode 0x%08x: RTM_ALWAYS_ABORT vs RTM mismatch\n",

This isn't really firmware, is it? At least I wouldn't call microcode
(assuming that's where the bad behavior is rooted) firmware.

> +                       boot_cpu_data.x86, boot_cpu_data.x86_model,
> +                       boot_cpu_data.x86_mask, this_cpu(cpu_sig).rev);
> +
> +                setup_clear_cpu_cap(X86_FEATURE_RTM_ALWAYS_ABORT);

Instead of the "goto" below, wouldn't it be better to also force
has_rtm_always_abort to false along with this, thus skipping the
setup_force_cpu_cap(X86_FEATURE_RTM_ALWAYS_ABORT) further down? That would
leave things a little less awkward flow-wise, imo. The one thing not
becoming clear from the commentary above is whether cpu_has_tsx_ctrl might
be true, and hence RTM/HLE still becoming (wrongly) set, if done that way.

Jan

> +                setup_clear_cpu_cap(X86_FEATURE_TSX_FORCE_ABORT);
> +
> +                if ( opt_tsx < 0 )
> +                    opt_tsx = -2;
> +
> +                goto done_setup;
> +            }
> +
>              /*
>               * Probe for the June 2021 microcode which de-features TSX on
>               * client parts.  (Note - this is a subset of parts impacted by
> @@ -128,15 +169,8 @@ void tsx_init(void)
>               * read as zero if TSX_FORCE_ABORT.ENABLE_RTM has been set before
>               * we run.
>               */
> -            if ( !has_rtm_always_abort )
> -            {
> -                uint64_t val;
> -
> -                rdmsrl(MSR_TSX_FORCE_ABORT, val);
> -
> -                if ( val & TSX_ENABLE_RTM )
> -                    has_rtm_always_abort = true;
> -            }
> +            if ( val & TSX_ENABLE_RTM )
> +                has_rtm_always_abort = true;
>  
>              /*
>               * If no explicit tsx= option is provided, pick a default.
> @@ -191,6 +225,7 @@ void tsx_init(void)
>              setup_force_cpu_cap(X86_FEATURE_RTM);
>          }
>      }
> + done_setup:
>  
>      /*
>       * Note: MSR_TSX_CTRL is enumerated on TSX-enabled MDS_NO and later parts.
> 
> base-commit: 6117179dd99958e4ef2687617d12c9b15bdbae24