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? -- MST
