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

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On Tue, Jul 14, 2020 at 09:05:39PM +0200, Peter Zijlstra wrote:
> 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.

Ah sorry, my mistake yes I meant 'per-logical-processor' like Dave said.  I'll
update the commit message.

Ira



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