[MODERATED] Re: [PATCH 7/9] TAA 7

On Thu, Oct 24, 2019 at 08:20:42PM +0200, speck for Jiri Kosina wrote:
> On Thu, 24 Oct 2019, speck for Borislav Petkov wrote:
> 
> > "Disable TSX if the CPU is affected by the TSX Async Abort (TAA)
> > vulnerability and microcode provides a special MSR - TSX_CTRL_MSR -
> > which provides the required TSX control knobs. On MDS-affected parts
> > where VERW takes care of the TAA vulnerability, that controlling MSR is
> > not present and thus TSX cannot be disabled there."
> 
> This is true if you ignore hyperthreading.
> 
> On SMT systems, TSX disable is 100% complete mitigation, while VERW
> clearing is not.

So why is our default this then?

static enum taa_mitigations taa_mitigation __ro_after_init = TAA_MITIGATION_VERW;

and we only do the TAA_MITIGATION_TSX_DISABLED thing only if TSX has
been disabled earlier?

Because of those MDS_NO=0 machines which don't get the TSX_CTRL MSR so
that TSX cannot be disabled there?

Are some of those machines SMT?

Because if so, we *must* disable SMT unconditionally to mitigate TAA
completely there... methinks.

-- 
Regards/Gruss,
    Boris.

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