On Mon, 2019-06-10 at 12:52 -0700, Dave Hansen wrote: > On 6/10/19 12:38 PM, Yu-cheng Yu wrote: > > > > When an application starts, its highest stack address is determined. > > > > It uses that as the maximum the bitmap needs to cover. > > > > > > Huh, I didn't think we ran code from the stack. ;) > > > > > > Especially given the way that we implemented the new 5-level-paging > > > address space, I don't think that expecting code to be below the stack > > > is a good universal expectation. > > > > Yes, you make a good point. However, allowing the application manage the > > bitmap > > is the most efficient and flexible. If the loader finds a legacy lib is > > beyond > > the bitmap can cover, it can deal with the problem by moving the lib to a > > lower > > address; or re-allocate the bitmap. > > How could the loader reallocate the bitmap and coordinate with other > users of the bitmap? Assuming the loader actually chooses to re-allocate, it can copy the old bitmap over to the new before doing the switch. But, I agree, the other choice is easier; the loader can simply put the lib at lower address. AFAIK, the loader does not request high address in mmap(). > > > If the loader cannot allocate a big bitmap to cover all 5-level > > address space (the bitmap will be large), it can put all legacy lib's > > at lower address. We cannot do these easily in the kernel. > > This is actually an argument to do it in the kernel. The kernel can > always allocate the virtual space however it wants, no matter how large. > If we hide the bitmap behind a kernel API then we can put it at high > 5-level user addresses because we also don't have to worry about the > high bits confusing userspace. We actually tried this. The kernel needs to reserve the bitmap space in the beginning for every CET-enabled app, regardless of actual needs. On each memory request, the kernel then must consider a percentage of allocated space in its calculation, and on systems with less memory this quickly becomes a problem.
