RE: [PATCH] x86/resctrl: Require force for L3 CAT on early Skylake-X

[Luck, Tony]

> When L3 CAT gets enabled with rdt=l3cat on Skylake-X stepping 3,
> consecutive kexec boots hang for me during secondary socket bringup.
> Cold boots on the same machine work.

This is troubling. I thought that the problems on Skylake were simply that in some circumstances
the CLOSID cache mask would not be honored so that data would spill into cache ways that
were supposed to be reserved for other users. I did not expect that the system could hang
with L3 cat enabled.

Your fix seems quite complex.  Perhaps something like this?

	case INTEL_SKYLAKE_X:
		if (boot_cpu_data.x86_stepping == 3)
			setup_clear_cpu_cap(X86_FEATURE_CAT_L3);
		if (boot_cpu_data.x86_stepping <= 4)
			set_rdt_options("!cmt,!mbmtotal,!mbmlocal,!l3cat");
		else
			set_rdt_options("!l3cat");
		fallthrough;

-Tony