On Fri, Jun 04, 2021 at 04:27:39PM +0100, Marc Zyngier wrote:
> > > Plus, I've found this nugget:
> > >
> > > <quote
> > > max_pinned_vmids = NUM_USER_VMIDS - num_possible_cpus() - 2;
> > > </quote>
> > >
> > > What is this "- 2"? My hunch is that it should really be "- 1" as VMID
> > > 0 is reserved, and we have no equivalent of KPTI for S2.
> >
> > I think this is more related to the "pinned vmid" stuff and was borrowed from
> > the asid_update_limit() fn in arch/arm64/mm/context.c. But I missed the
> > comment that explains the reason behind it. It says,
> >
> > ---x---
> > /*
> > * There must always be an ASID available after rollover. Ensure that,
> > * even if all CPUs have a reserved ASID and the maximum number of ASIDs
> > * are pinned, there still is at least one empty slot in the ASID map.
> > */
> > max_pinned_asids = num_available_asids - num_possible_cpus() - 2;
> > ---x---
> >
> > So this is to make sure we will have at least one VMID available
> > after rollover in case we have pinned the max number of VMIDs. I
> > will include that comment to make it clear.
>
> That doesn't really explain the -2. Or is that that we have one for
> the extra empty slot, and one for the reserved?
>
> Jean-Philippe?
Yes, -2 is for ASID#0 and the extra empty slot. A comment higher in
asids_update_limit() hints at that, but it could definitely be clearer
/*
* Expect allocation after rollover to fail if we don't have at least
* one more ASID than CPUs. ASID #0 is reserved for init_mm.
*/
Thanks,
Jean
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