Re: [PATCH] virtio_pci: do not mutate caller irq_affinity.pre_vectors

On Wed, Aug 05, 2026 at 01:37:01AM -0400, Michael S. Tsirkin wrote:
> "tmp" in what sense? if we can come up with a sensible name, just
> "affinity" will do.
>
> We do not need a comment arguing with previous version of the code.
>
> just assign to desc, instead?

Thanks, will do. I will post a fresh [PATCH v2] as a new standalone
mail (not threaded under this one) with:
- a local "affinity" copy
- that comment dropped
- desc = &affinity, and keep passing desc

On the virtio_input thread you also asked whether this is real or
theoretical for stable. For this virtio_pci patch: real when the caller
passes irq_affinity and MSI-X policy falls back.

vp_find_vqs() retries EACH -> SHARED_SLOW -> SHARED. A failed
vp_request_msix_vectors() currently does desc->pre_vectors++ in place,
so a later successful attempt sees a polluted pre_vectors and gets the
wrong affinity layout. That fallback path is unconditional in the
driver.

I have not collected a userspace failure log yet. Please let me know if
you still want Cc: stable on the v2.

Thanks,
Xiong