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

[Will Deacon]

On Fri, Nov 17, 2017 at 04:19:35PM -0800, Stephen Boyd wrote:
> On 11/17, Will Deacon wrote:
> > 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/<20171110193058.BECA7D88@xxxxxxxxxxxxxxxxxx>
> > 
> > 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