Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two buses

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Sep 01, 2015 at 12:47:36PM +0800, Jason Wang wrote:
> 
> 
> On 09/01/2015 12:31 PM, Michael S. Tsirkin wrote:
> > On Tue, Sep 01, 2015 at 11:33:43AM +0800, Jason Wang wrote:
> >>
> >> On 08/31/2015 07:33 PM, Michael S. Tsirkin wrote:
> >>> On Mon, Aug 31, 2015 at 04:03:59PM +0800, Jason Wang wrote:
> >>>>>
> >>>>> On 08/31/2015 03:29 PM, Michael S. Tsirkin wrote:
> >>>>>>>>>>>>>>> Thinking more about this, invoking the 0-length write after
> >>>>>>>>>>>>>>>>>>>>>>> the != 0 length one would be better: it would mean we only
> >>>>>>>>>>>>>>>>>>>>>>> handle the userspace MMIO like this.
> >>>>>>>>>>>>>>> Right.
> >>>>>>>>>>>>>>>
> >>>>>>>>>>> Using current unittest. This patch is about 2.9% slower than before, and
> >>>>>>>>>>> invoking 0-length write after is still 1.1% slower (mmio-datamatch-eventfd).
> >>>>>>>>>>>
> >>>>>>>>>>> /patch/result/-+%/
> >>>>>>>>>>> /base/2957/0/
> >>>>>>>>>>> /V3/3043/+2.9%/
> >>>>>>>>>>> /V3+invoking != 0 length first/2990/+1.1%/
> >>>>>>>>>>>
> >>>>>>>>>>> So looks like the best method is not searching KVM_FAST_MMIO_BUS during
> >>>>>>>>>>> KVM_MMIO_BUS. Instead, let userspace to register both datamatch and
> >>>>>>>>>>> wildcard in this case. Does this sound good to you?
> >>>>>>> No - we can't change userspace.
> >>>>> Actually, the change was as simple as following. So I don't get the
> >>>>> reason why.
> >>> Because it's too late - we committed to a specific userspace ABI
> >>> when this was merged in kernel, we must maintain it.
> >> Ok ( Though I don't think it has real users for this now because it was
> >> actually broken).
> > It actually worked most of the time - you only trigger a use after free
> > on deregister.
> >
> 
> It doesn't work for amd and intel machine without ept.

I thought it does :(

> >>> Even if I thought yours is a good API (and I don't BTW - it's exposing
> >>> internal implementation details) it's too late to change it.
> >> I believe we should document the special treatment in kernel of zero
> >> length mmio eventfd in api.txt? If yes, is this an exposing? If not, how
> >> can userspace know the advantages of this and use it? For better API,
> >> probably we need another new flag just for fast mmio and obsolete
> >> current one by failing the assigning for zero length mmio eventfd.
> > I sent a patch to update api.txt already as part of
> > kvm: add KVM_CAP_IOEVENTFD_PF capability.
> > I should probably split it out.
> >
> > Sorry, I don't think the api change you propose makes sense - just fix the
> > crash in the existing one.
> >
> 
> Ok, so I believe the fix should go:
> 
> - having two ioeventfds when we want to assign zero length mmio eventfd

You mean the in-kernel data structures?

> - change the kvm_io_bus_sort_cmp() and can handle zero length correctly

This one's for amd/non ept, right? I'd rather we implemented the
fast mmio optimization for these.

> What's your thought?