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. -- 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
