[MODERATED] Re: ***UNCHECKED*** Re: [PATCH v5 08/11] TAAv5 8

On Tue, Oct 15, 2019 at 10:35:04PM +0200, speck for Jiri Kosina wrote:
> On Tue, 15 Oct 2019, speck for Jiri Kosina wrote:
> 
> > > Maybe I'm missing something.  Isn't there going to be a ucode update for
> > > MDS_NO parts, which does the verw buffer clearing?  In that case there's
> > > no need to disable TSX, and instead the verw mitigation could be used,
> > > if desired.
> > 
> > My understanding was that MDS_NO CPUs will only get ucode update that 
> > exposes TSX control MSR, and nothing else.

I'm looking at a Cinco slide deck from Intel, where page 11 says that
CPUs with MDS_NO=1 will get a microcode update to provide both VERW
buffer clearing support and the new MSR interface to disable TSX.

> > > AFAICT, the patch allows to set the default to tsx=auto, which disables 
> > > TSX on *all* vulnerable parts, not just the MDS_NO ones.  I don't see 
> > > how that would prevent user regressions.
> > > 
> > > It sounds like maybe you're suggesting something else, that TSX should
> > > only be disabled on vulnerable MDS_NO parts?
> > 
> > OK, let me take a look at the code again. I definitely thought that's what 
> > 'auto' indeed does.
> 
> OK, so you are right and I misunderstood the logic in the code, sorry. 
> 
> Then the only purpose of 'auto' really is getting TSX enabled on future 
> CPUs which would eventually have ARCH_CAP_TAA_NO=1; so pretty useless for 
> preventing regressions.
> 
> So yeah, I agree, 'auto' is actually useless to prevent regressions, and I 
> believe we want some other 'auto' (*), which would actually disable TSX 
> only if (X86_BUG_TAA && !MD_CLEAR), agreed?

Based on the above-mentioned slide, I don't see how

  X86_BUG_TAA && !MD_CLEAR

is a realistic scenario, if the user has updated their microcode.

> (*) I'd actually prefer to convert the current 'auto' to this new 
>     semantics; it'll keep TSX enabled on future CPUs without X86_BUG_TAA,
>     and it'll prevent regressions in unnecessary cases.

I'm also not sure I see the point of the existing tsx=auto option,
unless we can identify who's actually going to use it.

-- 
Josh