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? -- 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
