Re: [RFC PATCH v2 6/6] KVM: SVM: Add CET features to supported_xss

Tom Lendacky
2023-06-26T16:46:07Z

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