[MODERATED] Re: ...

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

On Tue, Oct 22, 2019 at 12:10:31AM -0400, speck for Jon Masters wrote:
> Subject: Re: [PATCH v7 07/10] TAAv7 7

> On 10/21/19 4:29 PM, speck for Pawan Gupta wrote:
> > From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> > Subject: [PATCH v7 07/10] x86/tsx: Add "auto" option to TSX cmdline parameter
> > 
> > Platforms which are not affected by X86_BUG_TAA may want the TSX feature
> > enabled. Add "auto" option to the TSX cmdline parameter. When tsx=auto
> > disable TSX when X86_BUG_TAA is present, otherwise enable TSX.
> 
> Earlier, you do this:
> 
> +	if (!(ia32_cap & ARCH_CAP_TAA_NO) &&
> +	    (boot_cpu_has(X86_FEATURE_RTM) ||
> +	     (ia32_cap & ARCH_CAP_TSX_CTRL_MSR)))
> +		setup_force_cpu_bug(X86_BUG_TAA);
> 
> Per the other discussion, I think you want to double check if tsx=auto
> is doing what folks want it to do, because currently I think auto still
> has the semantics of turning off TSX on everything, rather than just
> those cases where a VERW mitigation won't suffice/is in use.

tsx=auto can only disable TSX on current CPUs(MDS_NO=1) because older
CPUs(MDS_NO=0) wont get TSX_CTRL_MSR.

On older CPUs tsx_init() would simply do nothing because of lack of TSX_CTRL. 

	void __init tsx_init(void)
	{
		if (!tsx_ctrl_is_supported())
			return;
	[...]

> 
> (see Thomas's update to the "Re: [PATCH v5 08/11] TAAv5 8" diagram)

  MDS_NO        MD_CLEAR        TSX_CTRL_MSR
    0             0                 0           Vulnerable (needs ucode)
    0             1                 0           MDS and TAA mitigated via VERW
    1             1                 0           MDS fixed, TAA vulnerable if TSX enabled
                                                because MD_CLEAR has no meaning and
                                                VERW is not guaranteed to clear buffers
    1             X                 1           MDS fixed, TAA can be mitigated by
                                                           VERW or TSX_CTRL_MSR

Thomas's table sums up the mitigation status.

> 
> It seems that it's a good time to double check if that's what everyone
> on the distro side is expecting, namely "tsx=auto will disable TSX
> automatically on those parts impacted by TAA for which VERW mitigation
> is not available".

Microcode update for MDS_NO=1 parts adds both TSX_CTRL and VERW
mitigation at the same time. It is for the OS to choose(tsx=) which
mitigation to deploy. 

For MDS_NO=0 CPUs:
	VERW mitigation will be deployed.
	tsx= is ineffective, for the lack of TSX_CTRL.

For MDS_NO=1 CPUs:
	If OS chooses tsx=on, VERW mitigation will be deployed.
	If OS chooses tsx=auto, TSX will be disabled.
	If OS chooses tsx=off, TSX will be disabled (for TAA_NO=1 as well).

More details on tsx=auto:

+----------+----------+----------------+---------------+--------------+-------------------+
|  MSR_IA32_ARCH_CAPABILITIES bits     |           Result with cmdline tsx=auto           |
+----------+----------+----------------+---------------+--------------+-------------------+
|  TAA_NO  |  MDS_NO  |  TSX_CTRL_MSR  |  VERW clears  |  TSX state   |  TAA mitigation   |
|          |          |                |  CPU buffers  | after bootup |                   |
+==========+==========+================+===============+==============+===================+
|    0     |    0     |       0        |      Yes      |  HW default  |    Same as MDS    |
+----------+----------+----------------+---------------+--------------+-------------------+
|    0     |    0     |       1        | Invalid case  | Invalid case |   Invalid case    |
+----------+----------+----------------+---------------+--------------+-------------------+
|    0     |    1     |       0        |      No       |  HW default  | Need ucode update |
+----------+----------+----------------+---------------+--------------+-------------------+
|    0     |    1     |       1        |      Yes      | TSX disabled |   TSX disabled    |
+----------+----------+----------------+---------------+--------------+-------------------+
|    1     |    X     |       1        |       X       | TSX enabled  |    None needed    |
+----------+----------+----------------+---------------+--------------+-------------------+

Note: MDS_NO=0 and TSX_CTRL_MSR=1 is an invalid case above, which
prevents tsx=auto from disabling TSX in older CPUs.

>This seems to reduce the set where we end up
> disabling TSX essentially to a few very recent processors (e.g.
> CascadeLake some second gen, Bx stepping?). If that's what we are all
> expecting/are planning to do, I suspect Red Hat would go with tsx=auto
> as a default since the set of impacted processors can be documented.
> 
> Anyway, auto isn't right yet.

Below change isn't required as per the above explanation, but I can add
it if adds clarity. Or maybe cover with code comments.

diff --git a/arch/x86/kernel/cpu/tsx.c b/arch/x86/kernel/cpu/tsx.c
index 0969e6e9dff3..7b567ca266c9 100644
--- a/arch/x86/kernel/cpu/tsx.c
+++ b/arch/x86/kernel/cpu/tsx.c
@@ -70,7 +70,8 @@ static bool __init tsx_ctrl_is_supported(void)
 
 static enum tsx_ctrl_states x86_get_tsx_auto_mode(void)
 {
-	if (boot_cpu_has_bug(X86_BUG_TAA))
+	if (boot_cpu_has_bug(X86_BUG_TAA) &&
+	    (ARCH_CAP_MDS_NO & x86_read_arch_cap_msr()))
 		return TSX_CTRL_DISABLE;
 
 	return TSX_CTRL_ENABLE;