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

[Michael S. Tsirkin]

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 28.2.3.1).
> >
> >	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,
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)
> >>>  		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.
> >
> >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.

> 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.

-- 
MST
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