Re: [RFC PATCH 04/15] x86/pks: Preserve the PKRS MSR on context switch

[Peter Zijlstra]

On Tue, Jul 14, 2020 at 11:53:22AM -0700, Ira Weiny wrote:
> On Tue, Jul 14, 2020 at 10:27:01AM +0200, Peter Zijlstra wrote:
> > On Tue, Jul 14, 2020 at 12:02:09AM -0700, ira.weiny@xxxxxxxxx wrote:
> > > From: Ira Weiny <ira.weiny@xxxxxxxxx>
> > > 
> > > The PKRS MSR is defined as a per-core register.  This isolates memory
> > > access by CPU.  Unfortunately, the MSR is not preserved by XSAVE.
> > > Therefore, We must preserve the protections for individual tasks even if
> > > they are context switched out and placed on another cpu later.
> > 
> > This is a contradiction and utter trainwreck.
> 
> I don't understand where there is a contradiction?  Perhaps I should have said
> the MSR is not XSAVE managed vs 'preserved'?

You're stating the MSR is per-*CORE*, and then continue to talk about
per-task state.

We've had a bunch of MSRs have exactly that problem recently, and it's
not fun. We're not going to do that again.