Messages in this thread | | | Subject | Re: [PATCH RFC 1/3] vmx: allow ioeventfd for EPT violations | From | Xiao Guangrong <> | Date | Mon, 31 Aug 2015 10:53:58 +0800 |
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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.
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