Re: [PATCH] KVM: VMX: remove LFENCE in vmx_spec_ctrl_restore_host()

[Jon Kohler]

> On Jun 1, 2023, at 12:23 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> 
> On Wed, May 31, 2023 at 06:24:29PM -0700, Pawan Gupta wrote:
> 
> ## 2023-05-31
>> On Thu, Jun 01, 2023 at 01:50:48AM +0100, Andrew Cooper wrote:
>>> On 01/06/2023 1:42 am, Josh Poimboeuf wrote:
>>>> So each LFENCE has a distinct purpose.  That said, there are no indirect
>>>> branches or unbalanced RETs between them.
>>> 
>>> How lucky are you feeling?
>>> 
>>> You're in C at this point, which means the compiler could have emitted a
>>> call to mem{cpy,cmp}() in place of a simple assignment/comparison.
>> 
>> Moving the second LFENCE to the else part of WRMSR should be possible?
>> So that the serialization can be achived either by WRMSR or LFENCE. This
>> saves an LFENCE when host and guest value of MSR_SPEC_CTRL differ.
> 
> Yes.  Though in practice it might not make much of a difference.  With
> wrmsr+lfence, the lfence has nothing to do so it might be almost
> instantaneous anyway.
> 
> -- 
> Josh

Coming back to this, what if we hoisted call vmx_spec_ctrl_restore_host above
FILL_RETURN_BUFFER, and dropped this LFENCE as I did here?

That way, we wouldn’t have to mess with the internal LFENCE in nospec-branch.h,
and that would act as the “final line of defense” LFENCE.

Would that be acceptable? Or does FILL_RETURN_BUFFER *need* to occur
before any sort of calls no matter what?

Thanks,
Jon