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? > Anyway just wanted to put the idea forward. > > Cheers, > Jérôme
