On Mon, Nov 16, 2020 at 10:10 AM Alexandre Chartre <alexandre.char...@oracle.com> wrote: > > > On 11/16/20 5:57 PM, Andy Lutomirski wrote: > > On Mon, Nov 16, 2020 at 6:47 AM Alexandre Chartre > > <alexandre.char...@oracle.com> wrote: > >> > >> When entering the kernel from userland, use the per-task PTI stack > >> instead of the per-cpu trampoline stack. Like the trampoline stack, > >> the PTI stack is mapped both in the kernel and in the user page-table. > >> Using a per-task stack which is mapped into the kernel and the user > >> page-table instead of a per-cpu stack will allow executing more code > >> before switching to the kernel stack and to the kernel page-table. > > > > Why? > > When executing more code in the kernel, we are likely to reach a point > where we need to sleep while we are using the user page-table, so we need > to be using a per-thread stack. > > > I can't immediately evaluate how nasty the page table setup is because > > it's not in this patch. > > The page-table is the regular page-table as introduced by PTI. It is just > augmented with a few additional mapping which are in patch 11 (x86/pti: > Extend PTI user mappings). > > > But AFAICS the only thing that this enables is sleeping with user > > pagetables. > > That's precisely the point, it allows to sleep with the user page-table. > > > Do we really need to do that? > > Actually, probably not with this particular patchset, because I do the > page-table > switch at the very beginning and end of the C handler. I had some code where I > moved the page-table switch deeper in the kernel handler where you > definitively > can sleep (for example, if you switch back to the user page-table before > exit_to_user_mode_prepare()). > > So a first step should probably be to not introduce the per-task PTI > trampoline stack, > and stick with the existing trampoline stack. The per-task PTI trampoline > stack can > be introduced later when the page-table switch is moved deeper in the C > handler and > we can effectively sleep while using the user page-table.
Seems reasonable. Where is the code that allocates and frees these stacks hiding? I think I should at least read it.
