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

From: Stephen Boyd
Date: Fri Nov 17 2017 - 19:19:50 EST

On 11/17, Will Deacon wrote:
> Hi all,
> This patch series implements something along the lines of KAISER for arm64:
> 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:
> and the topic was recently covered by LWN (currently subscriber-only):
> 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.

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