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

[Dave Hansen]

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.