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Date:   Mon, 23 Sep 2019 11:15:58 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Andrea Arcangeli <aarcange@redhat.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Peter Xu <peterx@redhat.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from
 vmx.c exit handlers
Message-ID: <20190923181558.GI18195@linux.intel.com>
References: <20190920212509.2578-1-aarcange@redhat.com>
 <20190920212509.2578-16-aarcange@redhat.com>
 <87o8zb8ik1.fsf@vitty.brq.redhat.com>
 <20190923163746.GE18195@linux.intel.com>
 <24dc5c23-eed8-22db-fd15-5a165a67e747@redhat.com>
 <20190923174244.GA19996@redhat.com>
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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.