Messages in this thread | | | Subject | Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address | From | Jason Wang <> | Date | Fri, 8 Mar 2019 17:15:59 +0800 |
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On 2019/3/8 上午11:45, Jerome Glisse wrote: > On Thu, Mar 07, 2019 at 10:43:12PM -0500, Michael S. Tsirkin wrote: >> On Thu, Mar 07, 2019 at 10:40:53PM -0500, Jerome Glisse wrote: >>> On Thu, Mar 07, 2019 at 10:16:00PM -0500, Michael S. Tsirkin wrote: >>>> On Thu, Mar 07, 2019 at 09:55:39PM -0500, Jerome Glisse wrote: >>>>> On Thu, Mar 07, 2019 at 09:21:03PM -0500, Michael S. Tsirkin wrote: >>>>>> On Thu, Mar 07, 2019 at 02:17:20PM -0500, Jerome Glisse wrote: >>>>>>>> It's because of all these issues that I preferred just accessing >>>>>>>> userspace memory and handling faults. Unfortunately there does not >>>>>>>> appear to exist an API that whitelists a specific driver along the lines >>>>>>>> of "I checked this code for speculative info leaks, don't add barriers >>>>>>>> on data path please". >>>>>>> Maybe it would be better to explore adding such helper then remapping >>>>>>> page into kernel address space ? >>>>>> I explored it a bit (see e.g. thread around: "__get_user slower than >>>>>> get_user") and I can tell you it's not trivial given the issue is around >>>>>> security. So in practice it does not seem fair to keep a significant >>>>>> optimization out of kernel because *maybe* we can do it differently even >>>>>> better :) >>>>> Maybe a slightly different approach between this patchset and other >>>>> copy user API would work here. What you want really is something like >>>>> a temporary mlock on a range of memory so that it is safe for the >>>>> kernel to access range of userspace virtual address ie page are >>>>> present and with proper permission hence there can be no page fault >>>>> while you are accessing thing from kernel context. >>>>> >>>>> So you can have like a range structure and mmu notifier. When you >>>>> lock the range you block mmu notifier to allow your code to work on >>>>> the userspace VA safely. Once you are done you unlock and let the >>>>> mmu notifier go on. It is pretty much exactly this patchset except >>>>> that you remove all the kernel vmap code. A nice thing about that >>>>> is that you do not need to worry about calling set page dirty it >>>>> will already be handle by the userspace VA pte. It also use less >>>>> memory than when you have kernel vmap. >>>>> >>>>> This idea might be defeated by security feature where the kernel is >>>>> running in its own address space without the userspace address >>>>> space present. >>>> Like smap? >>> Yes like smap but also other newer changes, with similar effect, since >>> the spectre drama. >>> >>> Cheers, >>> Jérôme >> Sorry do you mean meltdown and kpti? > Yes all that and similar thing. I do not have the full list in my head. > > Cheers, > Jérôme
Yes, address space of kernel its own is the main motivation of using vmap here.
Thanks
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