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

Vitaly Kuznetsov
2019-09-23T09:32:04Z

Andrea Arcangeli <aarcange@redhat.com> writes:

> It's enough to check the exit value and issue a direct call to avoid
> the retpoline for all the common vmexit reasons.
>
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 24 ++++++++++++++++++++++--
> 1 file changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index a6e597025011..9aa73e216df2 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -5866,9 +5866,29 @@ static int vmx_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 handle_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 handle_halt(vcpu);
> + else if (exit_reason == EXIT_REASON_PAUSE_INSTRUCTION)
> + return handle_pause(vcpu);
> + else if (exit_reason == EXIT_REASON_MSR_READ)
> + return handle_rdmsr(vcpu);
> + else if (exit_reason == EXIT_REASON_CPUID)
> + return handle_cpuid(vcpu);
> + else if (exit_reason == EXIT_REASON_EPT_MISCONFIG)
> + return handle_ept_misconfig(vcpu);
> +#endif
> return kvm_vmx_exit_handlers[exit_reason](vcpu);

I agree with the identified set of most common vmexits, however, this
still looks a bit random. Would it be too much if we get rid of
kvm_vmx_exit_handlers completely replacing this code with one switch()?

> - else {
> + } else {
> vcpu_unimpl(vcpu, "vmx: unexpected exit reason 0x%x\n",
> exit_reason);
> dump_vmcs();

--
Vitaly