Messages in this thread | | | Date | Mon, 26 Jun 2023 11:45:56 -0500 | Subject | Re: [RFC PATCH v2 6/6] KVM: SVM: Add CET features to supported_xss | From | Tom Lendacky <> |
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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, Tom
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