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

[Marc Zyngier <maz@kernel.org>]

On Wed, 26 May 2021 17:10:24 +0100,
Sandor Bodo-Merle <sbodomerle@gmail.com> wrote:
> 
> [1  <text/plain; UTF-8 (7bit)>]
> 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.
> [2 0001-PCI-iproc-fix-the-base-vector-number-allocation-for-.patch <text/x-diff; UTF-8 (base64)>]
> 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);
>  

This looks reasonable. However, this doesn't change the issue that you
have with SMP systems and Multi-MSI. I'd like to see a more complete
patch (disabling Multi-MSI on SMP, at the very least).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.