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. The user can get to keep both pieces if they've decided that this was a good thing to try. ~Andrew -- 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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