Re: [PATCH v2 0/3] virtio-gpu: fix blob unmapping sequence

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Apr 15, 2025 at 07:46:14PM +0100, Alex Bennée wrote:
> Manos Pitsidianakis <manos.pitsidianakis@linaro.org> 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.
> 
> Should I (or MST?) pull these into a tree for 10.0 or should they be
> grabbed for when the tree opens with a Cc qemu-stable?

QEMU 10.0.0-rc4 has already been tagged. No further patches will be
merged unless there is a show-stopper (build failure, security issue).

Please Cc qemu-stable so this can be merged for 10.0.1. Thanks!

> 
> >
> > 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
> >
> > v1 to v2:
> >   - Add patch by Alex to prevent double-free when FlatView is destroyed 
> >     from RCU thread.
> >
> > Alex Bennée (1):
> >   hw/display: re-arrange memory region tracking
> >
> > Manos Pitsidianakis (2):
> >   virtio-gpu: fix hang under TCG when unmapping blob
> >   virtio-gpu: refactor async blob unmapping
> >
> >  include/exec/memory.h         |  1 +
> >  hw/display/virtio-gpu-virgl.c | 60 ++++++++++++++++++++---------------
> >  2 files changed, 35 insertions(+), 26 deletions(-)
> >
> >
> > base-commit: 56c6e249b6988c1b6edc2dd34ebb0f1e570a1365
> 
> -- 
> Alex Bennée
> Virtualisation Tech Lead @ Linaro
> 

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