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Message-ID: <55E5603E.9070409@redhat.com> Date: Tue, 1 Sep 2015 16:22:22 +0800 From: Jason Wang <jasowang@...hat.com> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: gleb@...nel.org, pbonzini@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, cornelia.huck@...ibm.com Subject: Re: [PATCH V3 2/3] kvm: don't register wildcard MMIO EVENTFD on two buses 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? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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