linux-kernel - Re: [PATCH v4 2/2] arm64: PCI: hv: Add support for Hyper-V vPCI

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Date:   Wed, 10 Nov 2021 13:26:11 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Sunil Muthuswamy <sunilmut@...ux.microsoft.com>
Cc:     kys@...rosoft.com, haiyangz@...rosoft.com, sthemmin@...rosoft.com,
        wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        lorenzo.pieralisi@....com, robh@...nel.org, kw@...ux.com,
        bhelgaas@...gle.com, arnd@...db.de, sunilmut@...rosoft.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v4 2/2] arm64: PCI: hv: Add support for Hyper-V vPCI

On Wed, 10 Nov 2021 13:20:32 +0000,
Marc Zyngier <maz@...nel.org> wrote:
> 
> On Tue, 09 Nov 2021 22:14:20 +0000,
> Sunil Muthuswamy <sunilmut@...ux.microsoft.com> wrote:
> > 
> > From: Sunil Muthuswamy <sunilmut@...rosoft.com>
> > 
> > Add support for Hyper-V vPCI for arm64 by implementing the arch specific
> > interfaces. Introduce an IRQ domain and chip specific to Hyper-v vPCI that
> > is based on SPIs. The IRQ domain parents itself to the arch GIC IRQ domain
> > for basic vector management.
> > 
> > Signed-off-by: Sunil Muthuswamy <sunilmut@...rosoft.com>
> > ---
> > In v2, v3 & v4:
> >  Changes are described in the cover letter.
> > 
> >  arch/arm64/include/asm/hyperv-tlfs.h |   9 ++
> >  drivers/pci/Kconfig                  |   2 +-
> >  drivers/pci/controller/Kconfig       |   2 +-
> >  drivers/pci/controller/pci-hyperv.c  | 207 ++++++++++++++++++++++++++-
> >  4 files changed, 217 insertions(+), 3 deletions(-)
> 
> [...]
> 
> > +static int hv_pci_vec_irq_domain_activate(struct irq_domain *domain,
> > +					  struct irq_data *irqd, bool reserve)
> > +{
> > +	static int cpu;
> > +
> > +	/*
> > +	 * Pick a cpu using round-robin as the irq affinity that can be
> > +	 * temporarily used for composing MSI from the hypervisor. GIC
> > +	 * will eventually set the right affinity for the irq and the
> > +	 * 'unmask' will retarget the interrupt to that cpu.
> > +	 */
> > +	if (cpu >= cpumask_last(cpu_online_mask))
> > +		cpu = 0;
> > +	cpu = cpumask_next(cpu, cpu_online_mask);
> > +	irq_data_update_effective_affinity(irqd, cpumask_of(cpu));
> 
> The mind boggles.
> 
> Let's imagine a single machine. cpu_online_mask only has bit 0 set,

single *CPU* machine

> and nr_cpumask_bits is 1. This is the first run, and cpu is 1:

cpu is *obviously* 0:

>
> 	cpu = cpumask_next(cpu, cpu_online_mask);
> 
> cpu is now set to 1. Which is not a valid CPU number, but a valid
> return value indicating that there is no next CPU as it is equal to
> nr_cpumask_bits. cpumask_of(cpu) will then diligently return crap,
> which you carefully store into the irq descriptor. The IRQ subsystem
> thanks you.
> 
> The same reasoning applies to any number of CPUs, and you obviously
> never checked what any of this does :-(. As to what the behaviour is
> when multiple CPUs run this function in parallel, let's not even
> bother (locking is overrated).
> 
> Logic and concurrency issues aside, why do you even bother setting
> some round-robin affinity if all you need is to set *something* so
> that a hypervisor message can be composed? Why not use the first
> online CPU? At least it will be correct.

Everything else holds.

	M.

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