Messages in this thread | | | Date | Tue, 16 Nov 2021 11:27:09 +0100 | Subject | Re: [PATCH 16/39] irqdomain: Make normal and nomap irqdomains exclusive | From | Cédric Le Goater <> |
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Hello Marc,
>> This patch is breaking the POWER9/POWER10 XIVE driver (these are not >> old PPC systems :) on machines sharing the same LSI HW IRQ. For instance, >> a linux KVM guest with a virtio-rng and a virtio-balloon device. In that >> case, Linux creates two distinct IRQ mappings which can lead to some >> unexpected behavior. > > Either the irq domain translates, or it doesn't. If the driver creates > a nomap domain, and yet expects some sort of translation to happen, > then the driver is fundamentally broken. And even without that: how do > you end-up with a single HW interrupt having two mappings? > >> A fix to go forward would be to change the XIVE IRQ domain to use a >> 'Tree' domain for reverse mapping and not the 'No Map' domain mapping. >> I will keep you updated for XIVE. > > I bet there is a bit more to it. From what you are saying above, > something rather ungodly is happening in the XIVE code.
It's making progress.
This change in irq_find_mapping() is what 'breaks' XIVE :
+ if (irq_domain_is_nomap(domain)) { + if (hwirq < domain->revmap_size) { + data = irq_domain_get_irq_data(domain, hwirq); + if (data && data->hwirq == hwirq) + return hwirq; + } + + return 0;
With the introduction of IRQ_DOMAIN_FLAG_NO_MAP, the revmap_tree lookup is skipped and the previously mapped IRQ is not found. XIVE was relying on a side effect of irq_domain_set_mapping() which is not true anymore.
I guess the easiest fix for 5.14 and 5.15 (in which was introduced MSI domains) is to change the XIVE IRQ domain to a domain tree. Since the HW can handle 1MB interrupts, this looks like a better choice for the driver.
Thanks,
C.
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