Re: [PATCH 15/24] x86/mm: Allow flushing for future ASID switches

[Peter Zijlstra]

On Thu, Nov 30, 2017 at 11:00:06AM -0800, Dave Hansen wrote:
> On 11/30/2017 10:55 AM, Peter Zijlstra wrote:
> >> __flush_tlb() does a flushing CR3 write that flushes the current PCID.
> >> If we need other PCIDs flushed, we have to do it via the
> >> tlb_flush_shared_nonglobals() mechanism.
> > But the thing is, you _cannot_ have PCID enabled in that branch.
> 
> Is there some hardware limitation that I'm missing?

No, but in setup_pcid() we explicit clear PCID if !PGE.

And as you said, there isn't actually any real hardware that has this
combination, so who cares.