On 11/20/2017 09:21 AM, Thomas Gleixner wrote: >> +KAISER logically keeps a "copy" of the page tables which unmap >> +the kernel while in userspace. The kernel manages the page >> +tables as normal, but the "copying" is done with a few tricks >> +that mean that we do not have to manage two full copies. >> +The first trick is that for any any new kernel mapping, we >> +presume that we do not want it mapped to userspace. That means >> +we normally have no copying to do. We only copy the kernel >> +entries over to the shadow in response to a kaiser_add_*() >> +call which is rare. > When KAISER is enabled the kernel manages two page tables for the kernel > mappings. The regular page table which is used while executing in kernel > space and a shadow copy which only contains the mapping entries which are > required for the kernel-userspace transition. These mappings have to be > copied into the shadow page tables explicitely with the kaiser_add_*() > functions.
This misses a few important points that I think the original text touches on. I gave it another go: > Page Table Management > ===================== > > When KAISER is enabled, the kernel manages two sets of page > tables. The first copy is very similar to what would be present > for a kernel without KAISER. This includes a complete mapping of > userspace that the kernel can use for things like copy_to_user(). > > The second (shadow) is used when running userspace and mirrors the > mapping of userspace present in the kernel copy. It maps a only > the kernel data needed to enter and exit the kernel. > > The shadow is populated by the kaiser_add_*() functions. Only > kernel data which has been explicity mapped will appear in the > shadow copy. These calls are rare at runtime. > > For a new userspace mapping, the kernel makes the entries in its > page tables like normal. The only difference is when the kernel > makes entries in the top (PGD) level. In addition to setting the > entry in the main kernel PGD, a copy if the entry is made in the > shadow PGD. > > For user space mappings the kernel creates an entry in the kernel > PGD and the same entry in the shadow PGD, so the underlying page > table to which the PGD entry points is shared down to the PTE > level. This leaves a single, shared set of userspace page tables > to manage. One PTE to lock, one set set of accessed bits, dirty > bits, etc...
