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Date:   Wed, 19 Jul 2023 22:36:58 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sean Christopherson <seanjc@google.com>
Cc:     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>
Subject: Re: [PATCH v3 00/21] Enable CET Virtualization
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On Wed, Jul 19, 2023 at 12:41:47PM -0700, Sean Christopherson wrote:

> My understanding is that PL[0-2]_SSP are used only on transitions to the
> corresponding privilege level from a *different* privilege level.  That means
> KVM should be able to utilize the user_return_msr framework to load the host
> values.  Though if Linux ever supports SSS, I'm guessing the core kernel will
> have some sort of mechanism to defer loading MSR_IA32_PL0_SSP until an exit to
> userspace, e.g. to avoid having to write PL0_SSP, which will presumably be
> per-task, on every context switch.
> 
> But note my original wording: **If that's necessary**
> 
> If nothing in the host ever consumes those MSRs, i.e. if SSS is NOT enabled in
> IA32_S_CET, then running host stuff with guest values should be ok.  KVM only
> needs to guarantee that it doesn't leak values between guests.  But that should
> Just Work, e.g. KVM should load the new vCPU's values if SHSTK is exposed to the
> guest, and intercept (to inject #GP) if SHSTK is not exposed to the guest.
> 
> And regardless of what the mechanism ends up managing SSP MSRs, it should only
> ever touch PL0_SSP, because Linux never runs anything at CPL1 or CPL2, i.e. will
> never consume PL{1,2}_SSP.

To clarify, Linux will only use SSS in FRED mode -- FRED removes CPL1,2.