Re: [PATCH RFC 1/3] vmx: allow ioeventfd for EPT violations

[Xiao Guangrong <guangrong.xiao@linux.intel.com>]

On 08/30/2015 05:12 PM, Michael S. Tsirkin wrote:
> Even when we skip data decoding, MMIO is slightly slower
> than port IO because it uses the page-tables, so the CPU
> must do a pagewalk on each access.
>
> This overhead is normally masked by using the TLB cache:
> but not so for KVM MMIO, where PTEs are marked as reserved
> and so are never cached.
>
> As ioeventfd memory is never read, make it possible to use
> RO pages on the host for ioeventfds, instead.

I like this idea.

> The result is that TLBs are cached, which finally makes MMIO
> as fast as port IO.

What does "TLBs are cached" mean? Even after applying the patch
no new TLB type can be cached.

>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>   arch/x86/kvm/vmx.c | 5 +++++
>   1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 9d1bfd3..ed44026 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5745,6 +5745,11 @@ static int handle_ept_violation(struct kvm_vcpu *vcpu)
>   		vmcs_set_bits(GUEST_INTERRUPTIBILITY_INFO, GUEST_INTR_STATE_NMI);
>
>   	gpa = vmcs_read64(GUEST_PHYSICAL_ADDRESS);
> +	if (!kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
> +		skip_emulated_instruction(vcpu);
> +		return 1;
> +	}
> +

I am afraid that the common page fault entry point is not a good place to do the
work. Would move it to kvm_handle_bad_page()? The different is the workload of
fast_page_fault() is included but it's light enough and MMIO-exit should not be
very frequent, so i think it's okay.