Re: [PATCH v10 00/15] Linux RISC-V AIA Support

[Anup Patel]

On Fri, Oct 20, 2023 at 2:17 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
>
> Thanks for the quick reply!
>
> Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
>
> > On Thu, Oct 19, 2023 at 7:13 PM Björn Töpel <bjorn@xxxxxxxxxx> wrote:
> >>
> >> Hi Anup,
> >>
> >> Anup Patel <apatel@xxxxxxxxxxxxxxxx> writes:
> >>
> >> > The RISC-V AIA specification is ratified as-per the RISC-V international
> >> > process. The latest ratified AIA specifcation can be found at:
> >> > https://github.com/riscv/riscv-aia/releases/download/1.0/riscv-interrupts-1.0.pdf
> >> >
> >> > At a high-level, the AIA specification adds three things:
> >> > 1) AIA CSRs
> >> >    - Improved local interrupt support
> >> > 2) Incoming Message Signaled Interrupt Controller (IMSIC)
> >> >    - Per-HART MSI controller
> >> >    - Support MSI virtualization
> >> >    - Support IPI along with virtualization
> >> > 3) Advanced Platform-Level Interrupt Controller (APLIC)
> >> >    - Wired interrupt controller
> >> >    - In MSI-mode, converts wired interrupt into MSIs (i.e. MSI generator)
> >> >    - In Direct-mode, injects external interrupts directly into HARTs
> >>
> >> Thanks for working on the AIA support! I had a look at the series, and
> >> have some concerns about interrupt ID abstraction.
> >>
> >> A bit of background, for readers not familiar with the AIA details.
> >>
> >> IMSIC allows for 2047 unique MSI ("msi-irq") sources per hart, and
> >> each MSI is dedicated to a certain hart. The series takes the approach
> >> to say that there are, e.g., 2047 interrupts ("lnx-irq") globally.
> >> Each lnx-irq consists of #harts * msi-irq -- a slice -- and in the
> >> slice only *one* msi-irq is acutally used.
> >>
> >> This scheme makes affinity changes more robust, because the interrupt
> >> sources on "other" harts are pre-allocated. On the other hand it
> >> requires to propagate irq masking to other harts via IPIs (this is
> >> mostly done up setup/tear down). It's also wasteful, because msi-irqs
> >> are hogged, and cannot be used.
> >>
> >> Contemporary storage/networking drivers usually uses queues per core
> >> (or a sub-set of cores). The current scheme wastes a lot of msi-irqs.
> >> If we instead used a scheme where "msi-irq == lnx-irq", instead of
> >> "lnq-irq = {hart 0;msi-irq x , ... hart N;msi-irq x}", there would be
> >> a lot MSIs for other users. 1-1 vs 1-N. E.g., if a storage device
> >> would like to use 5 queues (5 cores) on a 128 core system, the current
> >> scheme would consume 5 * 128 MSIs, instead of just 5.
> >>
> >> On the plus side:
> >> * Changing interrupts affinity will never fail, because the interrupts
> >>   on each hart is pre-allocated.
> >>
> >> On the negative side:
> >> * Wasteful interrupt usage, and a system can potientially "run out" of
> >>   interrupts. Especially for many core systems.
> >> * Interrupt masking need to proagate to harts via IPIs (there's no
> >>   broadcast csr in IMSIC), and a more complex locking scheme IMSIC
> >>
> >> Summary:
> >> The current series caps the number of global interrupts to maximum
> >> 2047 MSIs for all cores (whole system). A better scheme, IMO, would be
> >> to expose 2047 * #harts unique MSIs.
> >>
> >> I think this could simplify/remove(?) the locking as well.
> >
> > Exposing 2047 * #harts unique MSIs has multiple issues:
> > 1) The irq_set_affinity() does not work for MSIs because each
> >      IRQ is not tied to a particular HART. This means we can't
> >      balance the IRQ processing load among HARTs.
>
> Yes, you can balance. In your code, each *active* MSI is still
> bound/active to a specific hard together with the affinity mask. In an
> 1-1 model you would still need to track the affinity mask, but the
> irq_set_affinity() would be different. It would try to allocate a new
> MSI from the target CPU, and then switch to having that MSI active.
>
> That's what x86 does AFAIU, which is also constrained by the # of
> available MSIs.
>
> The downside, as I pointed out, is that the set affinity action can
> fail for a certain target CPU.

Yes, irq_set_affinity() can fail for the suggested approach plus for
RISC-V AIA, one HART does not have access to other HARTs
MSI enable/disable bits so the approach will also involve IPI.

>
> > 2) All wired IRQs for APLIC MSI-mode will also target a
> >     fixed HART hence irq_set_affinity() won't work for wired
> >     IRQs as well.
>
> I'm not following here. Why would APLIC put a constraint here? I had a
> look at the specs, and I didn't see anything supporting the current
> scheme explicitly.

Lets say the number of APLIC wired interrupts  are greater than the
number of per-CPU IMSIC IDs. In this case, if all wired interrupts are
moved to a particular CPU then irq_set_affinity() will fail for some of
the wired interrupts.

>
> > 3) Contemporary storage/networking drivers which use per-core
> >      queues use irq_set_affinity() on queue IRQs to balance
> >      across cores but this will fail.
>
> Or via the the managed interrupts. But this is a non-issue, as pointed
> out in my reply to 1.
>
> > 4) HART hotplug breaks because kernel irq-subsystem can't
> >     migrate the IRQs (both MSIs and Wired) targeting HART X
> >     to another HART Y when the HART X goes down.
>
> Yes, we might end up in scenarios where we can't move to a certain
> target cpu, but I wouldn't expect that to be a common scenario.
>
> > The idea of treating per-HART MSIs as separate IRQs has
> > been discussed in the past.
>
> Aha! I tried to look for it in lore, but didn't find any. Could you
> point me to those discussions?

This was done 2 years back in the AIA TG meeting when we were
doing the PoC for AIA spec.

>
> > Also, the current approach is very similar to the ARM GICv3 driver
> > where ITS LPIs across CPUs are treated as single IRQ.
>
> I'm not familiar with the GIC. Is the GICv3 design similar to IMSIC? I
> had the impression that the GIC had a more advanced interrupt routing
> mechanism, than what IMSIC exposes. I think x86 APIC takes the 1-1
> approach (the folks on the To: list definitely knows! ;-)).

GIC has a per-CPU redistributor which handles LPIs. The MSIs are
taken by GIC ITS and forwarded as LPI to the redistributor of a CPU.

The GIC driver treats LPI numbering space as global and not per-CPU.
Also, the limit on maximum number of LPIs is quite high because LPI
INTID can be 32-bit wide.

>
> My concern is interrupts become a scarce resource with this
> implementation, but maybe my view is incorrect. I've seen bare-metal
> x86 systems (no VMs) with ~200 cores, and ~2000 interrupts, but maybe
> that is considered "a lot of interrupts".
>
> As long as we don't get into scenarios where we're running out of
> interrupts, due to the software design.
>

The current approach is simpler and ensures irq_set_affinity
always works. The limit of max 2047 IDs is sufficient for many
systems (if not all).

When we encounter a system requiring a large number of MSIs,
we can either:
1) Extend the AIA spec to support greater than 2047 IDs
2) Re-think the approach in the IMSIC driver

The choice between #1 and #2 above depends on the
guarantees we want for irq_set_affinity().

Regards,
Anup