On Tue, Sep 01, 2015 at 04:22:22PM +0800, Jason Wang wrote: > > > On 09/01/2015 02:54 PM, Michael S. Tsirkin wrote: > > 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? > > Yes. > > > > >> - 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. > > Agree, but we'd better fix it and backport it to stable first? I would say fix it upstream first. Worry about stable later. And I don't see a lot of value in adding a temporary hack - it's not too much work to just do the optimal thing directly. But I won't nack a temporary solution if you insist. -- MST -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
