Messages in this thread | | | Date | Thu, 11 Apr 2024 10:40:24 +0100 | Subject | Re: [PATCH v2] x86/mm: Don't disable INVLPG if "incomplete Global INVLPG flushes" is fixed by microcode | From | Andrew Cooper <> |
| |
On 11/04/2024 6:38 am, Xi Ruoyao wrote: > On Tue, 2024-04-09 at 08:56 +0100, Andrew Cooper wrote: >> On 09/04/2024 2:43 am, Sean Christopherson wrote: >>> On Mon, Apr 08, 2024, Michael Kelley wrote: >>>> From: Dave Hansen <dave.hansen@intel.com> Sent: Thursday, April 4, 2024 11:09 AM >>>>> On 4/4/24 10:48, Michael Kelley wrote: >>>>>> I agree one could argue that it is a hypervisor bug to present PCID to the guest >>>>>> in this situation. It's a lot cleaner to not have a guest be checking FMS and >>>>>> microcode versions. But whether that's practical in the real world, at least >>>>>> for Hyper-V, I don't know. What's the real impact of running with PCID while >>>>>> the flaw is still present? I don’t know the history here ... >>>>> There's a chance that INVLPG will appear ineffective. >>>>> >>>>> The bad sequence would go something like this: The kernel does the >>>>> INVLPG on a global mapping. Later, when switching PCIDs, the TLB entry >>>>> mysteriously reappears. No PCIDs switching means no mysterious >>>>> reappearance. >>>> Xi Ruoyao's patch identifies these errata: RPL042 and ADL063. In the links >>>> to the documents Xi provided, both of these errata have the following >>>> statement in the Errata Details section: >>>> >>>> This erratum does not apply in VMX non-root operation. It applies only >>>> when PCIDs are enabled and either in VMX root operation or outside >>>> VMX operation. >>>> >>>> I don't have deep expertise on the terminology here, but this sounds >>>> like it is saying the erratum doesn’t apply in a guest VM. Or am I >>>> misunderstanding? >>> Huh. My read of that is the same as yours. If that's the case, then it probably >>> makes sense to have KVM advertise support if PCID is available in hardware, even >>> if PCID is disabled by the host kernel. >> My reading is the same also. Seems like VMs are fine. > So... Should I sent a v6 with the hypervisor checking reverted [ i.e. > always enable PCID if boot_cpu_has(X86_FEATURE_HYPERVISOR) ]?
If Linux can see a hypervisor, then yes it should be safe to use PCID.
~Andrew
| |