Re: [PATCH v2] ACPI: Drop rcu usage for MMIO mappings

[Rafael J. Wysocki]

On Fri, Jun 5, 2020 at 6:39 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Fri, Jun 5, 2020 at 9:22 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> [..]
> > > The fix we are looking at now is to pre-map operation regions in a
> > > similar manner as the way APEI resources are pre-mapped. The
> > > pre-mapping would arrange for synchronize_rcu_expedited() to be elided
> > > on each dynamic mapping attempt. The other piece is to arrange for
> > > operation-regions to be mapped at their full size at once rather than
> > > a page at a time.
> >
> > However, if the RCU usage in ACPI OSL can be replaced with an rwlock,
> > some of the ACPICA changes above may not be necessary anymore (even
> > though some of them may still be worth making).
>
> I don't think you can replace the RCU usage in ACPI OSL and still
> maintain NMI lookups in a dynamic list.

I'm not sure what NMI lookups have to do with the issue at hand.

If acpi_os_{read|write}_memory() is used from NMI, that is a bug
already in there which is unrelated to the performance problem with
opregions.

> However, there are 3 solutions I see:
>
> - Prevent acpi_os_map_cleanup() from triggering at high frequency by
> pre-mapping and never unmapping operation-regions resources (internal
> discussion in progress)

Yes, that can be done, if necessary.

> - Prevent walks of the 'acpi_ioremaps' list (acpi_map_lookup_virt())
> from NMI context by re-writing the physical addresses in the APEI
> tables with pre-mapped virtual address, i.e. remove rcu_read_lock()
> and list_for_each_entry_rcu() from NMI context.

That sounds a bit convoluted to me.

> - Split operation-region resources into a separate mapping mechanism
> than APEI resources so that typical locking can be used for the
> sleepable resources and let the NMI accessible resources be managed
> separately.
>
> That last one is one we have not discussed internally, but it occurred
> to me when you mentioned replacing RCU.

So NMI cannot use acpi_os_{read|write}_memory() safely which you have
pointed out for a few times.

But even if NMI resources are managed separately, the others will
still not be sleepable (at least not all of them).

Cheers!