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

From: Jason Wang
Date: Tue Sep 01 2015 - 00:48:13 EST

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.

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

What's your thought?
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