On Mon, Sep 23, 2019 at 01:42:44PM -0400, Andrea Arcangeli wrote: > On Mon, Sep 23, 2019 at 06:53:10PM +0200, Paolo Bonzini wrote: > > On 23/09/19 18:37, Sean Christopherson wrote: > > >> Would it be too much if we get rid of > > >> kvm_vmx_exit_handlers completely replacing this code with one switch()? > > > Hmm, that'd require redirects for nVMX functions since they are set at > > > runtime. That isn't necessarily a bad thing. The approach could also be > > > used if Paolo's idea of making kvm_vmx_max_exit_handlers const allows the > > > compiler to avoid retpoline. > > > > But aren't switch statements also retpolin-ized if they use a jump table? > > See commit a9d57ef15cbe327fe54416dd194ee0ea66ae53a4. > > We disabled that feature or the kernel would potentially suffer the > downsides of the exit handlers through pointer to functions for every > switch statement in the kernel. > > In turn you can't make it run any faster by converting my "if" to a > "switch" at least the "if" can deterministic control the order of what > is more likely that we should also re-review, but the order of secondary > effect, the important thing is to reduce the retpolines to zero during > normal hrtimer guest runtime. On the flip side, using a switch for the fast-path handlers gives the compiler more flexibility to rearrange and combine checks. Of course that doesn't mean the compiler will actually generate faster code for our purposes :-) Anyways, getting rid of the retpolines is much more important.
