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

[Josh Poimboeuf]

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