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

[Michael S. Tsirkin]

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.

> > 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.

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