[v1,0/2] virtio-gpu: fix blob unmapping sequence

Message

Manos Pitsidianakis April 3, 2025, 12:17 p.m. UTC

A hang was observed when running a small kernel that exercised VIRTIO 
GPU under TCG. This is an edge-case and won't happen under typical 
conditions.

When unmapping a blob object, its MemoryRegion's freeing is deferred to 
the RCU thread. The hang's cause was determined to be a busy main loop 
not allowing for the RCU thread to run because the kernel did not setup 
any timers or had any interrupts on the way. While fixing the RCU thread 
to run even if the guest CPU spins is a solution, it's easier to fix the 
reason why the MemoryRegion isn't freed from the main loop instead.

While at it, also restructure the 3 stage cleanup to immediately respond 
to the guest if the MR happened to have had no other reference.

PS: The hang can be reproduced by running this unikernel with TCG 

https://git.codelinaro.org/manos.pitsidianakis/virtio-tests/-/tree/8c0ebe9395827e24aa5711186d499bf5de87cf63/virtio-test-suite

Manos Pitsidianakis (2):
  virtio-gpu: fix hang under TCG when unmapping blob
  virtio-gpu: refactor async blob unmapping

 hw/display/virtio-gpu-virgl.c | 37 ++++++++++++++++++++++++-----------
 1 file changed, 26 insertions(+), 11 deletions(-)


base-commit: 0adf626718bc0ca9c46550249a76047f8e45da15

Comments

Alex Bennée April 9, 2025, 10:32 a.m. UTC | #1

Manos Pitsidianakis <[e-mail]> writes:

> A hang was observed when running a small kernel that exercised VIRTIO 
> GPU under TCG. This is an edge-case and won't happen under typical 
> conditions.
>
> When unmapping a blob object, its MemoryRegion's freeing is deferred to 
> the RCU thread. The hang's cause was determined to be a busy main loop 
> not allowing for the RCU thread to run because the kernel did not setup 
> any timers or had any interrupts on the way. While fixing the RCU thread 
> to run even if the guest CPU spins is a solution, it's easier to fix the 
> reason why the MemoryRegion isn't freed from the main loop instead.
>
> While at it, also restructure the 3 stage cleanup to immediately respond 
> to the guest if the MR happened to have had no other reference.
>
> PS: The hang can be reproduced by running this unikernel with TCG 
>
> https://git.codelinaro.org/manos.pitsidianakis/virtio-tests/-/tree/8c0ebe9395827e24aa5711186d499bf5de87cf63/virtio-test-suite

Hmm these seems to regress the virtio-gpu tests: