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

[Josh Poimboeuf <jpoimboe@redhat.com>]

On Tue, Oct 15, 2019 at 11:14:03PM +0200, speck for Jiri Kosina wrote:
> On Tue, 15 Oct 2019, speck for Pawan Gupta wrote:
> 
> > tsx_ctrl_is_supported() checks for the TSX_CTRL MSR support and then 
> > only attempts at disabling TSX. MDS_NO=0 CPUs will not get the ucode 
> > update for TSX control, therefore tsx=auto wont cause regression on 
> > older CPUs.

So just to clarify, CPUs with TAA_BUG and MDS_NO=0 will *not* have
ARCH_CAP_TSX_CTRL_MSR?

I didn't see that important detail mentioned anywhere in the patches.

At the very least, the documentation for tsx=auto and tsx=off need to be
clarified, as they don't seem to mention any MDS_NO=0 contingencies.

> OK, that piece of information finally made it to make sense again :)
> 
> So I believe distros still want the option (Michal's patch) to default to 
> 'auto', so that actual heavy users of TSX will get the right thing once 
> they update their CPUs to !TAA_BUG ones, but it's less urgent that I 
> originally thought.

So if I understand correctly, you're postulating that distros want:

a) TAA_BUG && MDS_NO=0 => TSX on
b) TAA_BUG && MDS_NO=1 => TSX off
c) !TAA_BUG            => TSX on

How are you reaching that conclusion?  It seems horribly confusing for
TSX users, but again maybe I'm missing something.

It seems to me that "heavy users of TSX" would want tsx=on, no matter
what.  And so we would need to leave that as the default in order to not
break those users.

-- 
Josh