[PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)

On Fri, Nov 17, 2017 at 04:19:35PM -0800, Stephen Boyd wrote:

On 11/17, Will Deacon wrote:

quoted

Hi all,

This patch series implements something along the lines of KAISER for arm64:

  https://gruss.cc/files/kaiser.pdf

although I wrote this from scratch because the paper has some funny
assumptions about how the architecture works. There is a patch series
in review for x86, which follows a similar approach:

  http://lkml.kernel.org/r/[off-list ref]

and the topic was recently covered by LWN (currently subscriber-only):

  https://lwn.net/Articles/738975/

The basic idea is that transitions to and from userspace are proxied
through a trampoline page which is mapped into a separate page table and
can switch the full kernel mapping in and out on exception entry and
exit respectively. This is a valuable defence against various KASLR and
timing attacks, particularly as the trampoline page is at a fixed virtual
address and therefore the kernel text can be randomized independently.

The major consequences of the trampoline are:

  * We can no longer make use of global mappings for kernel space, so
    each task is assigned two ASIDs: one for user mappings and one for
    kernel mappings

  * Our ASID moves into TTBR1 so that we can quickly switch between the
    trampoline and kernel page tables

  * Switching TTBR0 always requires use of the zero page, so we can
    dispense with some of our errata workaround code.

  * entry.S gets more complicated to read

The performance hit from this series isn't as bad as I feared: things
like cyclictest and kernbench seem to be largely unaffected, although
syscall micro-benchmarks appear to show that syscall overhead is roughly
doubled, and this has an impact on things like hackbench which exhibits
a ~10% hit due to its heavy context-switching.

Do you have performance benchmark numbers on CPUs with the Falkor
errata? I'm interested to see how much the TLB invalidate hurts
heavy context-switching workloads on these CPUs.

I don't, but I'm also not sure what I can do about it if it's an issue.

Will