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. Thanks, Tom > > Thanks, > > tglx > -- 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
|