Hi JC, Thanks for posting this. On Fri, Oct 11, 2019 at 10:35:21AM +0000, Jayachandran Chandrasekharan Nair wrote: > These two patches are based on the work by Marc Zyngier and addresses > Cavium ThunderX2 erratum 219. > > This erratum (originally reported by ARM folks) is from an interesting > use of the prefetch instruction in the KPTI patchset. The prefetch > was done between a TTBR change and the corresponding ISB, and this > occasionally caused a crash on ThunderX2. > > The first patch removes the troublesome prefetch for ThunderX2. > The second patch addresses the case where the issue can be triggered > from a guest kernel. The workaround in this case is to trap TTBR > accesses by setting HCR_EL2.TVM for guests and doing the system > register update from EL2 in a fast path. FWIW, I was already planning to send the following to Linus: https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/log/?h=errata/tx2-219 so please base any changes on top of that branch. > Due to the nature of the erratum, the trap-and-emulate is only > needed when SMT is enabled. > > The overhead of trap-and-emulate is expected to be negligible on most > workloads. A command line option kvm-arm.vm_msr_trap has been > provided to override trapping on guest TTBR updates. This is to > address a very limited case where a user wants to run SMT enabled, > with a trustworthy guest kernel, and wants to avoid the performance > overhead associated with emulating the address translation register > changes. Do you have any performance data to show the impact of the workaround on non-kpti guests? I don't think we can justify the inclusion of a cmdline option for this without figures showing that it's really necessary. Otherwise, the "very limited case" really is a niche scenario where the CONFIG option can simply be disabled. Cheers, Will _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
