RE: [PATCH v2] x86/mm: Don't disable INVLPG if "incomplete Global INVLPG flushes" is fixed by microcode

[Michael Kelley]

From: Dave Hansen <dave.hansen@xxxxxxxxx> 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?

Michael