Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 26 Jun 2023 11:22:18 -0700
In-Reply-To: <a8bae991-c9af-ef45-20de-b9369f4dbf09@amd.com>
Mime-Version: 1.0
References: <20230524155339.415820-1-john.allen@amd.com> <20230524155339.415820-7-john.allen@amd.com>
 <161174d013dff42ddfd2950fe33a8054f45c223e.camel@intel.com>
 <ZINGaJnNJ54+klsD@johallen-workstation.lan> <9ef2faeaa38e667bd4daa8ee338d4cade452c76c.camel@intel.com>
 <ZJYaNSzup+yuYxNy@google.com> <52ea8386-8652-dd91-23de-9d35781cb131@amd.com>
 <ZJm64a7IKuSDS9bz@google.com> <a8bae991-c9af-ef45-20de-b9369f4dbf09@amd.com>
Message-ID: <ZJnXWmaN57fi7WbV@google.com>
Subject: Re: [RFC PATCH v2 6/6] KVM: SVM: Add CET features to supported_xss
From:   Sean Christopherson <seanjc@google.com>
To:     Tom Lendacky <thomas.lendacky@amd.com>
Cc:     Rick P Edgecombe <rick.p.edgecombe@intel.com>,
        "john.allen@amd.com" <john.allen@amd.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>
Content-Type: text/plain; charset="us-ascii"
Precedence: bulk

On Mon, Jun 26, 2023, Tom Lendacky wrote:
> On 6/26/23 11:28, Sean Christopherson wrote:
> > On Mon, Jun 26, 2023, Tom Lendacky wrote:
> > > On 6/23/23 17:18, Sean Christopherson wrote:
> > > > On Fri, Jun 09, 2023, Rick P Edgecombe wrote:
> > > > > Also, since the host might have CR4.CET set for its own reasons, if the host
> > > > > handled an exit with the the guests MSR_IA32_S_CET set it could suddenly be
> > > > > subjected to CET enforcement that it doesn't expect. Waiting to restore it
> > > > > until returning to the guest is too late.
> > > > > 
> > > > > At least that's the reasoning on the VMX side as I understand it
> > > > 
> > > > The APM doesn't come right out and say it, but I assume/hope that S_CET is saved
> > > > on VMRUN and loaded on #VMEXIT, i.e. is the same as VMX for all intents and
> > > > purposes.
> > > > 
> > > > The host save state definitely has a field for S_CET, and VMRUN documents that the
> > > > guest values are loaded, I just can't find anything in the APM that explicitly states
> > > > how host S_CET and friends are handled.  E.g. in theory, they could have been
> > > > shoved into VMSAVE+VMLOAD, though I very much doubt that's the case.
> > > 
> > > Yes, the host value is saved/restored on VMRUN/#VMEXIT. Anything that is in
> > > the VMCB Save Area (the non-SEV-ES save area) is fully virtualized (unless
> > > noted otherwise) and doesn't require special processing to save/restore the
> > > host values.
> > 
> > Would it makes sense to add a column in "Table B-2. VMCB Layout, State Save Area"
> > to specify whether a field is handled by VMRUN+#VMEXIT vs. VMLOAD+VMSAVE?  I can't
> > find anywhere in the APM where it explicitly states that VMRUN+#VMEXIT context
> > switches everything in the Save Area except the fields listed in "15.5.2 VMSAVE
> > and VMLOAD Instructions".
> > 
> > "15.5 VMRUN Instruction" kinda sorta covers that behavior, but the information is
> > either incomplete or stale, e.g. for host state it says "at least the following"
> > 
> >    Saving Host State. To ensure that the host can resume operation after #VMEXIT,
> >    VMRUN saves at least the following host state information:
> > 
> > but for guest state it says "the following"
> > 
> >    Loading Guest State. After saving host state, VMRUN loads the following guest
> >    state from the VMCB:
> > 
> > and then both provide incomplete lists of state.  A pedantic reading of the guest
> > case suggests that there's a large pile of state that *isn't* loaded, and the host
> > case isn't all that helpful because it's way too handwavy.
> 
> I'll communicate this feedback to the folks that update the APM volumes and
> see what can be done.

Thanks, much appreciated!