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

From: Michael S. Tsirkin
Date: Sun Aug 30 2015 - 05:13:17 EST

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

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
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)

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;
+ }
trace_kvm_page_fault(gpa, exit_qualification);

/* It is a write fault? */

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