Re: PCI MSI issue for maxcpus=1

[John Garry <john.garry@huawei.com>]

On 17/01/2022 09:14, Marc Zyngier wrote:
>> I guess that for managed interrupts, it shouldn't matter, as these
>> interrupts should only be used when the relevant CPUs come online.
>>
>> Would something like below help? Totally untested, as I don't have a
>> Multi-MSI capable device that I can plug in a GICv3 system (maybe I
>> should teach that to a virtio device...).

JFYI, NVMe PCI uses the same API (pci_alloc_irq_vectors_affinity()), but 
does not suffer from this issue - for maxcpus=1 the driver looks to only 
want 1x vector

> Actually, if the CPU online status doesn't matter for managed affinity
> interrupts, then the correct fix is this:
> 
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index d25b7a864bbb..af4e72a6be63 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -1624,7 +1624,7 @@ static int its_select_cpu(struct irq_data *d,
>   
>   		cpu = cpumask_pick_least_loaded(d, tmpmask);
>   	} else {
> -		cpumask_and(tmpmask, irq_data_get_affinity_mask(d), cpu_online_mask);
> +		cpumask_copy(tmpmask, irq_data_get_affinity_mask(d));
>   
>   		/* If we cannot cross sockets, limit the search to that node */
>   		if ((its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144) &&

That produces a warn:

[ 7.833025] ------------[ cut here ]------------
[ 7.837634] WARNING: CPU: 0 PID: 44 at 
drivers/irqchip/irq-gic-v3-its.c:298 valid_col+0x14/0x24
[ 7.846324] Modules linked in:
[ 7.849368] CPU: 0 PID: 44 Comm: kworker/0:3 Not tainted 5.16.0-dirty #119
[ 7.856230] Hardware name: Huawei D06 /D06, BIOS Hisilicon D06 UEFI RC0 
- V1.16.01 03/15/2019
[ 7.864740] Workqueue: events work_for_cpu_fn
[ 7.869088] pstate: 804000c9 (Nzcv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 7.876037] pc : valid_col+0x14/0x24
[ 7.879600] lr : its_build_mapti_cmd+0x84/0x90

...

[ 7.961007]  valid_col+0x14/0x24
[ 7.964223]  its_send_single_command+0x4c/0x150
[ 7.968741]  its_irq_domain_activate+0xc8/0x104
[ 7.973259]  __irq_domain_activate_irq+0x5c/0xac
[ 7.977865]  __irq_domain_activate_irq+0x38/0xac
[ 7.982471]  irq_domain_activate_irq+0x3c/0x64
[ 7.986902]  __msi_domain_alloc_irqs+0x1a8/0x2f4
[ 7.991507]  msi_domain_alloc_irqs+0x20/0x2c
[ 7.995764]  __pci_enable_msi_range+0x2ec/0x590
[ 8.000284]  pci_alloc_irq_vectors_affinity+0xe0/0x140
[ 8.005410]  hisi_sas_v3_probe+0x300/0xbe0
[ 8.009494]  local_pci_probe+0x44/0xb0
[ 8.013232]  work_for_cpu_fn+0x20/0x34
[ 8.016969]  process_one_work+0x1d0/0x354
[ 8.020966]  worker_thread+0x2c0/0x470
[ 8.024703]  kthread+0x17c/0x190
[ 8.027920]  ret_from_fork+0x10/0x20
[ 8.031485] ---[ end trace bb67cfc7eded7361 ]---

Apart from this, I assume that if another cpu comes online later in the 
affinity mask I would figure that we want to target the irq to that cpu 
(which I think we would not do here).

Cheers,
John