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

From: Xiao Guangrong
Date: Mon Aug 31 2015 - 09:29:25 EST

On 08/31/2015 07:27 PM, Michael S. Tsirkin wrote:
On Mon, Aug 31, 2015 at 04:32:52PM +0800, Xiao Guangrong wrote:

On 08/31/2015 03:46 PM, Michael S. Tsirkin wrote:
On Mon, Aug 31, 2015 at 10:53:58AM +0800, Xiao Guangrong wrote:

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.

The Intel manual says:
No guest-physical mappings or combined mappings are created with
information derived from EPT paging-structure entries that are not present
(bits 2:0 are all 0) or that are misconfigured (see Section

No combined mappings are created with information derived from guest
paging-structure entries that are not present or that set reserved bits.

Thus mappings that result in EPT violation are created, this makes
EPT violation preferable to EPT misconfiguration.

Hmm... but your logic completely bypasses page-table-installation, the page
table entry is nonpresent forever for eventfd memory.

As far as I can tell, not really: a non present page is not an EPT violation,

Actually, no.

There are two EPT VM-exit, one is EPT violation which is caused if EPT entry is
not present or the access permission is not enough. Another one is EPT misconfig
which is caused if EPT reserved bits is set.

so at the first write, the regular logic will trigger and install the PTE,
then instruction is re-executed and trigger an EPT violation.

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;
+ }

I am afraid that the common page fault entry point is not a good place to do the

Why isn't it?

1) You always do bus_write even if it is a read access. You can not assume that the
memory region can't be read by guest.

2) The workload of _bus_write is added for all kinds of page fault, normal #PF is fair
frequent than #PF happens on RO memory.

Normal PF is slow path: you need to hit disk to pull memory from swap,
etc etc. More importantly, it installs the PTE so the next access
does not cause an exit at all.

At some level that is the whole point of the patch: we are adding a fast
path option to what would normally be slow path only, so we aren't
slowing down anything important.

I have another question, the eventfd memory is never read by guest and it's always
a write MMIO VM-exit, why you build it on RO memslot? Why not just use normal MMIO page

3) It completely bypasses the logic of handing RO memslot.

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.

That was supposed to be a slow path, I doubt it'll work well without
major code restructuring.
IIUC by design everything that's not going through fast_page_fault
is supposed to be slow path that only happens rarely.

Do you have performance numbers which compare this patch and the way i figured out?

Not yet.

But in this case, the page stays read-only, we need a new fast path
through the code.

Another solution is making MMU recognise the RO region which is write-mostly, then
make the page table entry be reserved other than readonly.

Reserved results in EPT misconfiguration, so it's not cached.

See my comments above.

More addition: even if the EPT entry is cache, it is only a readonly permission in TLB
entry, this is useless to speed up write access.

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