linux-kernel - Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

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Date:   Mon, 23 Sep 2019 16:52:57 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 15/17] KVM: retpolines: x86: eliminate retpoline from
 vmx.c exit handlers

On Mon, Sep 23, 2019 at 07:43:07PM -0400, Andrea Arcangeli wrote:
> On Mon, Sep 23, 2019 at 02:24:35PM -0700, Sean Christopherson wrote:
> > An extra CALL+RET isn't going to be noticeable, especially on modern
> > hardware as the high frequency VMWRITE/VMREAD fields should hit the
> > shadow VMCS.
> 
> In your last email with regard to the inlining optimizations made
> possible by the monolithic KVM model you said "That'd likely save a
> few CALL/RET/JMP instructions", that kind of directly contradicts the
> above. I think neither one if taken at face value can be possibly
> measured. However the above only is relevant for nested KVM so I'm
> fine if there's an agreement that it's better to hide the nested vmx
> handlers in nested.c at the cost of some call/ret.

For the immediate exit case, eliminating the CALL/RET/JMP instructions
is a bonus.  The real goal was to eliminate the oddity of bouncing
through vendor code to invoke a one-line x86 function.  Having a separate
__kvm_request_immediate_exit() made sense when overwriting kvm_ops_x86, but
not so much when using direct calls.