linux-kernel - Re: [PATCH v3] s390/pci: fix CPU address in MSI for directed IRQ

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Date:   Fri, 27 Nov 2020 16:39:36 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] s390/pci: fix CPU address in MSI for directed IRQ

On Fri, 27 Nov 2020 11:08:10 +0100
Niklas Schnelle <schnelle@...ux.ibm.com> wrote:

> 
> 
> On 11/27/20 9:56 AM, Halil Pasic wrote:
> > On Thu, 26 Nov 2020 18:00:37 +0100
> > Alexander Gordeev <agordeev@...ux.ibm.com> wrote:
> > 
> >> The directed MSIs are delivered to CPUs whose address is
> >> written to the MSI message data. The current code assumes
> >> that a CPU logical number (as it is seen by the kernel)
> >> is also that CPU address.
> >>
> >> The above assumption is not correct, as the CPU address
> >> is rather the value returned by STAP instruction. That
> >> value does not necessarily match the kernel logical CPU
> >> number.
> >>
> >> Fixes: e979ce7bced2 ("s390/pci: provide support for CPU directed interrupts")
> >> Signed-off-by: Alexander Gordeev <agordeev@...ux.ibm.com>
> >> ---
> >>  arch/s390/pci/pci_irq.c | 14 +++++++++++---
> >>  1 file changed, 11 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/arch/s390/pci/pci_irq.c b/arch/s390/pci/pci_irq.c
> >> index 743f257cf2cb..75217fb63d7b 100644
> >> --- a/arch/s390/pci/pci_irq.c
> >> +++ b/arch/s390/pci/pci_irq.c
> >> @@ -103,9 +103,10 @@ static int zpci_set_irq_affinity(struct irq_data *data, const struct cpumask *de
> >>  {
> >>  	struct msi_desc *entry = irq_get_msi_desc(data->irq);
> >>  	struct msi_msg msg = entry->msg;
> >> +	int cpu_addr = smp_cpu_get_cpu_address(cpumask_first(dest));
> >>  
> >>  	msg.address_lo &= 0xff0000ff;
> >> -	msg.address_lo |= (cpumask_first(dest) << 8);
> >> +	msg.address_lo |= (cpu_addr << 8);
> >>  	pci_write_msi_msg(data->irq, &msg);
> >>  
> >>  	return IRQ_SET_MASK_OK;
> >> @@ -238,6 +239,7 @@ int arch_setup_msi_irqs(struct pci_dev *pdev, int nvec, int type)
> >>  	unsigned long bit;
> >>  	struct msi_desc *msi;
> >>  	struct msi_msg msg;
> >> +	int cpu_addr;
> >>  	int rc, irq;
> >>  
> >>  	zdev->aisb = -1UL;
> >> @@ -287,9 +289,15 @@ int arch_setup_msi_irqs(struct pci_dev *pdev, int nvec, int type)
> >>  					 handle_percpu_irq);
> >>  		msg.data = hwirq - bit;
> >>  		if (irq_delivery == DIRECTED) {
> >> +			if (msi->affinity)
> >> +				cpu = cpumask_first(&msi->affinity->mask);
> >> +			else
> >> +				cpu = 0;
> >> +			cpu_addr = smp_cpu_get_cpu_address(cpu);
> >> +
> > 
> > I thin style wise, I would prefer keeping the ternary operator instead
> > of rewriting it as an if-then-else, i.e.:
> >                         cpu_addr = smp_cpu_get_cpu_address(msi->affinity ?      
> >                                 cpumask_first(&msi->affinity->mask) : 0);
> > but either way:
> > 
> > Reviewed-by: Halil Pasic <pasic@...ux.ibm.com> 
> 
> Thanks for your review, lets keep the if/else its certainly not less
> readable even if it may be less pretty.
> 
> Found another thing (not directly in the touched code) but I'm now
> wondering about. In zpci_handle_cpu_local_irq()
> we do
> 	struct airq_iv *dibv = zpci_ibv[smp_processor_id()];
> 
> does that also need to use some _address() variant? If it does that
> then dicatates that the CPU addresses must start at 0.
> 

I didn't go to the bottom of this, but my understanding is that it
does not need a _address() variant. What we need is, probably, the
mapping between the 'id' and 'address' being a stable one.

Please notice that cpu_enable_directed_irq() is called on each cpu. That
establishes the mapping/relationship between the id and the address,
as the machine cares for the address, and cpu_enable_directed_irq()
cares for the id:
static void __init cpu_enable_directed_irq(void *unused)                        
{                                                                               
        union zpci_sic_iib iib = {{0}};                                         
                                                                                
        iib.cdiib.dibv_addr = (u64) zpci_ibv[smp_processor_id()]->vector;       
                                                                                
        __zpci_set_irq_ctrl(SIC_IRQ_MODE_SET_CPU, 0, &iib);                     
        zpci_set_irq_ctrl(SIC_IRQ_MODE_D_SINGLE, PCI_ISC);                      
}

Now were the id <-> address mapping to change, we would be in trouble. If
that's possible, I don't know. My guess is that it would require cpu hot
unplug. Niklas, are you familiar with that stuff? Should we ask, Heiko
and Vasily?

Regards,
Halil

> > 
> >>  			msg.address_lo = zdev->msi_addr & 0xff0000ff;
> >> -			msg.address_lo |= msi->affinity ?
> >> -				(cpumask_first(&msi->affinity->mask) << 8) : 0;
> >> +			msg.address_lo |= (cpu_addr << 8);
> >> +
> >>  			for_each_possible_cpu(cpu) {
> >>  				airq_iv_set_data(zpci_ibv[cpu], hwirq, irq);
> >>  			}
> >