Re: pcie-iproc-msi.c: Bug in Multi-MSI support?

[Sandor Bodo-Merle <sbodomerle@gmail.com>]

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The following patch addresses the allocation issue - but indeed - wont
fix the atomicity of IRQ affinity in this driver (but the majority of
our product relies on single core SOCs; we also use a dual-core SOC
also - but we don't change the initial the IRQ affinity).

On Wed, May 26, 2021 at 9:57 AM Marc Zyngier <maz@kernel.org> wrote:
>
> On Tue, 25 May 2021 18:27:54 +0100,
> Ray Jui <ray.jui@broadcom.com> wrote:
> >
> > On 5/24/2021 3:37 AM, Marc Zyngier wrote:
> > > On Thu, 20 May 2021 18:11:32 +0100,
> > > Ray Jui <ray.jui@broadcom.com> wrote:
> > >>
> > >> On 5/20/2021 7:22 AM, Sandor Bodo-Merle wrote:
>
> [...]
>
> > >> I guess I'm not too clear on what you mean by "multi-MSI interrupts
> > >> needs to be aligned to number of requested interrupts.". Would you be
> > >> able to plug this into the above explanation so we can have a more clear
> > >> understanding of what you mean here?
> > >
> > > That's a generic PCI requirement: if you are providing a Multi-MSI
> > > configuration, the base vector number has to be size-aligned
> > > (2-aligned for 2 MSIs, 4 aligned for 4, up to 32), and the end-point
> > > supplies up to 5 bits that are orr-ed into the base vector number,
> > > with a *single* doorbell address. You effectively provide a single MSI
> > > number and a single address, and the device knows how to drive 2^n MSIs.
> > >
> > > This is different from MSI-X, which defines multiple individual
> > > vectors, each with their own doorbell address.
> > >
> > > The main problem you have here (other than the broken allocation
> > > mechanism) is that moving an interrupt from one core to another
> > > implies moving the doorbell address to that of another MSI
> > > group. This isn't possible for Multi-MSI, as all the MSIs must have
> > > the same doorbell address. As far as I can see, there is no way to
> > > support Multi-MSI together with affinity change on this HW, and you
> > > should stop advertising support for this feature.
> > >
> >
> > I was not aware of the fact that multi-MSI needs to use the same
> > doorbell address (aka MSI posted write address?). Thank you for helping
> > to point it out. In this case, yes, like you said, we cannot possibly
> > support both multi-MSI and affinity at the same time, since supporting
> > affinity requires us to move from one to another event queue (and irq)
> > that will have different doorbell address.
> >
> > Do you think it makes sense to do the following by only advertising
> > multi-MSI capability in the single CPU core case (detected runtime via
> > 'num_possible_cpus')? This will at least allow multi-MSI to work in
> > platforms with single CPU core that Sandor and Pali use?
>
> I don't think this makes much sense. Single-CPU machines are an oddity
> these days, and I'd rather you simplify this (already pretty
> complicated) driver.
>
> > > There is also a more general problem here, which is the atomicity of
> > > the update on affinity change. If you are moving an interrupt from one
> > > CPU to the other, it seems you change both the vector number and the
> > > target address. If that is the case, this isn't atomic, and you may
> > > end-up with the device generating a message based on a half-applied
> > > update.
> >
> > Are you referring to the callback in 'irq_set_addinity" and
> > 'irq_compose_msi_msg'? In such case, can you help to recommend a
> > solution for it (or there's no solution based on such architecture)? It
> > does not appear such atomy can be enforced from the irq framework level.
>
> irq_compose_msi_msg() is only one part of the problem. The core of the
> issue is that the programming of the end-point is not atomic (you need
> to update a 32bit payload *and* a 64bit address).
>
> A solution to workaround it would be to rework the way you allocate
> the vectors, making them constant across all CPUs so that only the
> address changes when changing the affinity.
>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.

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From df31c9c0333ca4922b7978b30719348e368bea3c Mon Sep 17 00:00:00 2001
From: Sandor Bodo-Merle <sbodomerle@gmail.com>
Date: Wed, 26 May 2021 17:48:16 +0200
Subject: [PATCH] PCI: iproc: fix the base vector number allocation for Multi
 MSI
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Commit fc54bae28818 ("PCI: iproc: Allow allocation of multiple MSIs")
failed to reserve the proper number of bits from the inner domain.
Natural alignment of the base vector number was also not guaranteed.

Fixes: fc54bae28818 ("PCI: iproc: Allow allocation of multiple MSIs")
Reported-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Sandor Bodo-Merle <sbodomerle@gmail.com>
---
 drivers/pci/controller/pcie-iproc-msi.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git drivers/pci/controller/pcie-iproc-msi.c drivers/pci/controller/pcie-iproc-msi.c
index eede4e8f3f75..fa2734dd8482 100644
--- drivers/pci/controller/pcie-iproc-msi.c
+++ drivers/pci/controller/pcie-iproc-msi.c
@@ -252,18 +252,15 @@ static int iproc_msi_irq_domain_alloc(struct irq_domain *domain,
 
 	mutex_lock(&msi->bitmap_lock);
 
-	/* Allocate 'nr_cpus' number of MSI vectors each time */
-	hwirq = bitmap_find_next_zero_area(msi->bitmap, msi->nr_msi_vecs, 0,
-					   msi->nr_cpus, 0);
-	if (hwirq < msi->nr_msi_vecs) {
-		bitmap_set(msi->bitmap, hwirq, msi->nr_cpus);
-	} else {
-		mutex_unlock(&msi->bitmap_lock);
-		return -ENOSPC;
-	}
+	/* Allocate 'nr_irqs' multiplied by 'nr_cpus' number of MSI vectors each time */
+	hwirq = bitmap_find_free_region(msi->bitmap, msi->nr_msi_vecs,
+					order_base_2(msi->nr_cpus * nr_irqs));
 
 	mutex_unlock(&msi->bitmap_lock);
 
+	if (hwirq < 0)
+		return -ENOSPC;
+
 	for (i = 0; i < nr_irqs; i++) {
 		irq_domain_set_info(domain, virq + i, hwirq + i,
 				    &iproc_msi_bottom_irq_chip,
@@ -284,7 +281,8 @@ static void iproc_msi_irq_domain_free(struct irq_domain *domain,
 	mutex_lock(&msi->bitmap_lock);
 
 	hwirq = hwirq_to_canonical_hwirq(msi, data->hwirq);
-	bitmap_clear(msi->bitmap, hwirq, msi->nr_cpus);
+	bitmap_release_region(msi->bitmap, hwirq,
+			      order_base_2(msi->nr_cpus * nr_irqs));
 
 	mutex_unlock(&msi->bitmap_lock);
 
-- 
2.31.0