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Date:   Thu, 20 Jul 2023 10:09:47 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Pankaj Gupta <pankaj.gupta.linux@gmail.com>
Cc:     Sean Christopherson <seanjc@google.com>,
        Weijiang Yang <weijiang.yang@intel.com>, pbonzini@redhat.com,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org, rppt@kernel.org,
        binbin.wu@linux.intel.com, rick.p.edgecombe@intel.com,
        john.allen@amd.com, Chao Gao <chao.gao@intel.com>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>
Subject: Re: [PATCH v3 00/21] Enable CET Virtualization
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On Thu, Jul 20, 2023 at 10:03:58AM +0200, Peter Zijlstra wrote:

> > Trying to understand more what prevents SSS to enable in pre FRED, Is
> > it better #CP exception
> > handling with other nested exceptions?
> 
> SSS took the syscall gap and made it worse -- as in *way* worse.
> 
> To top it off, the whole SSS busy bit thing is fundamentally
> incompatible with how we manage to survive nested exceptions in NMI
> context.
> 
> Basically, the whole x86 exception / stack switching logic was already
> borderline impossible (consider taking an MCE in the early NMI path
> where we set up, but have not finished, the re-entrancy stuff), and

SSS

> pushed it over the edge and set it on fire.
> 
> And NMI isn't the only problem, the various new virt exceptions #VC and
> #HV are on their own already near impossible, adding SSS again pushes
> the whole thing into clear insanity.
> 
> There's a good exposition of the whole trainwreck by Andrew here:
> 
>   https://www.youtube.com/watch?v=qcORS8CN0ow
> 
> (that is, sorry for the youtube link, but Google is failing me in
> finding the actual Google Doc that talk is based on, or even the slide
> deck :/)
> 
> 
> 
> FRED solves all that by:
> 
>  - removing the stack gap, cc/ip/ss/sp/ssp/gs will all be switched
>    atomically and consistently for every transition.
> 
>  - removing the non-reentrant IST mechanism and replacing it with stack
>    levels
> 
>  - adding an explicit NMI latch
> 
>  - re-organising the actual shadow stacks and doing away with that busy
>    bit thing (I need to re-read the FRED spec on this detail again).
> 
> 
> 
> Crazy as we are, we're not touching legacy/IDT SSS with a ten foot pole,
> sorry.