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

On the virtio_input thread, Michael asked whether each of these patches
is a real or a theoretical issue for stable. For this virtio_pci patch:

Real, when affinity is requested and MSI-X policy falls back.

vp_find_vqs() retries EACH -> SHARED_SLOW -> SHARED. Each 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/IRQ-affinity failure log yet. Please
let me know if you still want Cc: stable on this one.

Thanks,
Xiong