* Andrew Cooper (andrew.cooper3@xxxxxxxxxx) wrote: > On 08/01/18 14:47, Tom Lendacky wrote: > > On 1/8/2018 5:10 AM, Thomas Gleixner wrote: > >> 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. > > Correct, the retpoline will work, it just takes more cycles. > > > >> That still does not help vs. rdtsc_ordered() and LFENCE_RDTSC ... > > Ok, I can add the read-back check before setting the feature flag(s). > > > > But... what about the case where the guest is a different family than > > hypervisor? If we're on, say, a Fam15h hypervisor but the guest is started > > as a Fam0fh guest where the MSR doesn't exist and LFENCE is supposed to be > > serialized? I'll have to do a rdmsr_safe() and only set the flag(s) if I > > can successfully read the MSR back and validate the bit. > > If your hypervisor is lying to you about the primary family, then all > bets are off. I don't expect there will be any production systems doing > this. It's not that an unusual thing to do on qemu/kvm - to specify the lowest common denominator of the set of CPUs in your data centre (for any one vendor); it does tend to get some weird combinations. Dave > The user can get to keep both pieces if they've decided that this was a > good thing to try. > > ~Andrew -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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