linux-kernel - Re: [PATCH v2 14/21] KVM:VMX: Add a synthetic MSR to allow userspace VMM to access GUEST

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Date:   Wed, 3 May 2023 17:08:02 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>,
        "Yang, Weijiang" <weijiang.yang@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "john.allen@....com" <john.allen@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC:     "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>
Subject: Re: [PATCH v2 14/21] KVM:VMX: Add a synthetic MSR to allow userspace
 VMM to access GUEST_SSP

On Fri, 2023-04-21 at 09:46 -0400, Yang Weijiang wrote:
> Introduce a host-only synthetic MSR, MSR_KVM_GUEST_SSP, so that the
> VMM
> can read/write the guest's SSP, e.g. to migrate CET state.  Use a
> synthetic
> MSR, e.g. as opposed to a VCPU_REG_, as GUEST_SSP is subject to the
> same
> consistency checks as the PL*_SSP MSRs, i.e. can share code.

It seems this is exposed to the guest? I'm thinking maybe it should not
be. IA32_PL0_SSP comes with some extra checks, so MSR_KVM_GUEST_SSP
seems a bit powerful. I think the guest doesn't need it either.