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 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?

>> ---
>> 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.

Jan