Re: [PATCH v4 1/4] x86/spec-ctrl: add logic to issue IBPB on exit to guest

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.12.2023 16:40, Roger Pau Monné wrote:
> On Mon, Dec 18, 2023 at 02:46:37PM +0100, Jan Beulich wrote:
>> On 18.12.2023 13:11, Roger Pau Monné wrote:
>>> Hello,
>>>
>>> I'm not as expert as Andrew in all the speculation stuff, but I will
>>> try to provide some feedback.
>>>
>>> On Tue, Feb 14, 2023 at 05:10:42PM +0100, Jan Beulich wrote:
>>>> In order to be able to defer the context switch IBPB to the last
>>>> possible point, add logic to the exit-to-guest paths to issue the
>>>> barrier there, including the "IBPB doesn't flush the RSB/RAS"
>>>> workaround. Since alternatives, for now at least, can't nest, emit JMP
>>>> to skip past both constructs where both are needed. This may be more
>>>> efficient anyway, as the sequence of NOPs is pretty long.
>>>
>>> Could you elaborate on the reason why deferring the IBPB to the exit
>>> to guest path is helpful?  So far it just seem to make the logic more
>>> complex without nay justification (at least in the changelog).
>>
>> I've added "(to leave behind as little as possible)" ahead of the 1st
>> comma - is that sufficient, do you think?
> 
> Please bear with me, but I'm still uncertain.
> 
> Even if IBRS is not enabled, and such indirect branch predictions are
> at the same predictor mode, how would that be of any use to a guest?
> My understanding was that the attacker is the one that has to control
> the indirect branch predictor contents in order to attack the
> hypervisor or other guests.

Right; see my later reply.

>>>> ---
>>>> I have to admit that I'm not really certain about the placement of the
>>>> IBPB wrt the MSR_SPEC_CTRL writes. For now I've simply used "opposite of
>>>> entry".
>>>
>>> Maybe it would easier to just add the MSR_PRED_CMD PRED_CMD_IBPB write
>>> to the vmcs MSR load list?
>>>
>>> It's a one-time only AFAICT, as you would only want to do this for
>>> context-switch AFAICT.
>>
>> That would be a back and forth of adding and removing the MSR to/from
>> that list then, which I'm not convinced is helpful. With these special
>> MSRs I would further be uncertain as to their effect when used via one
>> of these lists.
> 
> Hm, we do seem to already use MSR_PRED_CMD with such lists, so I would
> assume they work just fine.

Ah, yes, I forgot about that. Still it would be a back and forth if we
wanted the MSR on the list only after a context switch, but not anymore
for later VM entry. Also iirc these lists are VMX-only?

Jan

> Anyway, was mostly a recommendation towards clarity, because I think
> the return to guest context assembly is getting rather convoluted, and
> it's IMO critical for it to be easy to follow.
> 
> Thanks, Roger.