Re: [PATCH 11/13] KVM: retpolines: x86: eliminate retpoline from vmx.c exit handlers

Paolo Bonzini
2019-11-05T10:22:58Z

On 04/11/19 23:59, Andrea Arcangeli wrote:
> It's enough to check the exit value and issue a direct call to avoid
> the retpoline for all the common vmexit reasons.
>
> Of course CONFIG_RETPOLINE already forbids gcc to use indirect jumps
> while compiling all switch() statements, however switch() would still
> allow the compiler to bisect the case value. It's more efficient to
> prioritize the most frequent vmexits instead.
>
> The halt may be slow paths from the point of the guest, but not
> necessarily so from the point of the host if the host runs at full CPU
> capacity and no host CPU is ever left idle.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index a6afa5f4a01c..582f837dc8c2 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5905,9 +5905,23 @@ int kvm_x86_handle_exit(struct kvm_vcpu *vcpu)
> }
>
> if (exit_reason < kvm_vmx_max_exit_handlers
> - && kvm_vmx_exit_handlers[exit_reason])
> + && kvm_vmx_exit_handlers[exit_reason]) {
> +#ifdef CONFIG_RETPOLINE
> + if (exit_reason == EXIT_REASON_MSR_WRITE)
> + return kvm_emulate_wrmsr(vcpu);
> + else if (exit_reason == EXIT_REASON_PREEMPTION_TIMER)
> + return handle_preemption_timer(vcpu);
> + else if (exit_reason == EXIT_REASON_PENDING_INTERRUPT)
> + return handle_interrupt_window(vcpu);
> + else if (exit_reason == EXIT_REASON_EXTERNAL_INTERRUPT)
> + return handle_external_interrupt(vcpu);
> + else if (exit_reason == EXIT_REASON_HLT)
> + return kvm_emulate_halt(vcpu);
> + else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
> + return handle_ept_misconfig(vcpu);
> +#endif
> return kvm_vmx_exit_handlers[exit_reason](vcpu);
> - else {
> + } else {
> vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
> exit_reason);
> dump_vmcs();
>

Queued, thanks.

Paolo