On 04/01/2018 15:51, Andrew Cooper wrote: > Where have you got this idea from? Using IBPB on every mode switch > would be an insane overhead to take, and isn't necessary.
IIRC it started as a paranoia mode for AMD, but then we found out it was actually faster than IBRS on some Intel processor where IBRS performance was horrible. But I don't remember the details of the performance testing, sorry. Paolo > Also, remember that PTI and these mitigations are for orthogonal issues. > > Perhaps it is easiest to refer directly to the Xen SP2 mitigations and > my commentary of what is going on: > http://xenbits.xen.org/gitweb/?p=people/andrewcoop/xen.git;a=blob;f=xen/arch/x86/spec_ctrl.c;h=79aedf774a390293dfd564ce978500085344e305;hb=refs/heads/sp2-mitigations-v6.5#l192 > > With the GCC -mindirect-branch=thunk-external support, and microcode, > Xen will make a boot-time choice between using Retpoline, Lfence (which > is the better AMD option, and more performant than retpoline), or IBRS > on Skylake and newer processors where it is strictly necessary, as well > as using IBPB whenever available. > > It also supports virtualising IBRS for guest usage when the kernel has > chosen not to use it; a configuration I haven't seen in any of the Linux > patch series thusfar. > > ~Andrew >
