On Mon, 8 Jan 2018, Andrew Cooper wrote: > On 08/01/18 10:08, Thomas Gleixner wrote: > > On Sat, 6 Jan 2018, tip-bot for Tom Lendacky wrote: > > > >> Commit-ID: 0bf17c102177d5da9363bf8b1e4704b9996d5079 > >> Gitweb: https://git.kernel.org/tip/0bf17c102177d5da9363bf8b1e4704b9996d5079 > >> Author: Tom Lendacky <thomas.lendacky@xxxxxxx> > >> AuthorDate: Fri, 5 Jan 2018 10:07:56 -0600 > >> Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > >> CommitDate: Sat, 6 Jan 2018 21:57:40 +0100 > >> > >> x86/cpu/AMD: Use LFENCE_RDTSC instead of MFENCE_RDTSC > >> > >> With LFENCE now a serializing instruction, set the LFENCE_RDTSC > >> feature since the LFENCE instruction has less overhead than the > >> MFENCE instruction. > > Second thoughts on that. As pointed out by someone in one of the insane > > long threads: > > > > What happens if the kernel runs as a guest and > > > > - the hypervisor did not set the LFENCE to serializing on the host > > > > - the hypervisor does not allow writing MSR_AMD64_DE_CFG > > > > That would bring the guest into a pretty bad state or am I missing > > something essential here? > > What I did in Xen was to attempt to set it, then read it back and see. > If LFENCE still isn't serialising, using repoline is the only available > mitigation. > > My understanding from the folk at AMD is that retpoline is safe to use, > but has higher overhead than the LFENCE approach. That still does not help vs. rdtsc_ordered() and LFENCE_RDTSC ... Thanks, tglx
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