Re: [PATCH] x86: Check RTM_ALWAYS_ABORT for RTM [BZ #28033]

[Florian Weimer <fweimer@redhat.com>]

* H. J. Lu via Libc-alpha:

> From
>
> https://www.intel.com/content/www/us/en/support/articles/000059422/processors.html
>
> A new CPUID bit CPUID.07H.0H.EDX[11](RTM_ALWAYS_ABORT) will be enumerated,
> which is set to indicate to updated software that the loaded microcode is
> forcing RTM abort.
>
> 1. Add RTM_ALWAYS_ABORT to CPUID features.
> 2. Set RTM usable only if RTM_ALWAYS_ABORT isn't set.
> 3. Check RTM feature, instead of usability, against /proc/cpuinfo.

Maybe not that this fixes the string/tst-memchr-rtm etc. test cases
after a microcde update?

> diff --git a/manual/platform.texi b/manual/platform.texi
> index 4cd029cfad..8ec7f385e9 100644
> --- a/manual/platform.texi
> +++ b/manual/platform.texi
> @@ -525,6 +525,9 @@ capability.
>  @item
>  @code{RTM} -- RTM instruction extensions.
>  
> +@item
> +@code{RTM_ALWAYS_ABORT} -- Abort all transactions.

I think this means “Transactions always abort, making RTM unusable.”
(with unusable in both senses, !CPU_FEATURE_USABLE, and not useful).

> diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
> index a1d8d11cc4..d9093f11ac 100644
> --- a/sysdeps/x86/cpu-features.c
> +++ b/sysdeps/x86/cpu-features.c
> @@ -67,7 +67,6 @@ update_usable (struct cpu_features *cpu_features)
>    CPU_FEATURE_SET_USABLE (cpu_features, HLE);
>    CPU_FEATURE_SET_USABLE (cpu_features, BMI2);
>    CPU_FEATURE_SET_USABLE (cpu_features, ERMS);
> -  CPU_FEATURE_SET_USABLE (cpu_features, RTM);
>    CPU_FEATURE_SET_USABLE (cpu_features, RDSEED);
>    CPU_FEATURE_SET_USABLE (cpu_features, ADX);
>    CPU_FEATURE_SET_USABLE (cpu_features, CLFLUSHOPT);
> @@ -97,6 +96,9 @@ update_usable (struct cpu_features *cpu_features)
>    CPU_FEATURE_SET_USABLE (cpu_features, FSRCS);
>    CPU_FEATURE_SET_USABLE (cpu_features, PTWRITE);
>  
> +  if (!CPU_FEATURES_CPU_P (cpu_features, RTM_ALWAYS_ABORT))
> +    CPU_FEATURE_SET_USABLE (cpu_features, RTM);
> +
>  #if CET_ENABLED
>    CPU_FEATURE_SET_USABLE (cpu_features, IBT);
>    CPU_FEATURE_SET_USABLE (cpu_features, SHSTK);

Is some change necessary to copy RTM_ALWAYS_ABORT to USABLE as well?

Any idea why the microcode update doesn't just clear the RPM bit in
CPUID?  This is a bit awkward.

Thanks,
Florian