Re: [PATCH 0/4] vfio/virtio: Fix list_lock type and modernize locking

[Yishai Hadas <yishaih@nvidia.com>]

On 14/04/2026 23:06, Alex Williamson wrote:
> Jinhui Guo reported a mismatched spin_lock()/spin_unlock_irq() pair
> in virtiovf_read_device_context_chunk() where spin_unlock_irq() would
> unconditionally enable interrupts despite spin_lock() never having
> disabled them.  On closer inspection, the list_lock spinlock with IRQ
> disabling was copied from the mlx5 variant driver where a hardirq
> completion callback justifies it, but the virtio driver has no
> interrupt context usage of list_lock.  Patch 1 converts list_lock to
> a mutex, fixing the mismatch and aligning with peer vfio-pci variant
> drivers.

Alex,
How about staying with spin_lock but without the 'irq' variant, instead 
of replacing to mutex ?

The scope of the lock is very small which can fit spin.
We may potentially get a performance degradation compared to mutex as 
part of the hot path of STOP_COPY where this lock is used.

Note:
I don't see that other peer vfio-pci variant drivers maintain a list of 
buffers as of this driver, unless I missed that.

> Patch 2 converts the list_lock acquisitions to guard()/scoped_guard()
> where the lock scope aligns naturally with function or block scope.
> 

This patch might not be applicable if we'll stay with spin_lock.

> Patches 3 and 4 extend the same guard() conversion to the remaining
> two mutexes in the driver (migf->lock and bar_mutex).  These are
> relatively independent of the list_lock fix but complete the
> conversion across the driver.  Thanks,
> 

Those 2 patches seem fine to me.
Reviewed-by: Yishai Hadas <yishaih@nvidia.com>

Yishai

> Alex
> 
> Alex Williamson (4):
>    vfio/virtio: Convert list_lock from spinlock to mutex
>    vfio/virtio: Use guard() for list_lock where applicable
>    vfio/virtio: Use guard() for migf->lock where applicable
>    vfio/virtio: Use guard() for bar_mutex in legacy I/O
> 
>   drivers/vfio/pci/virtio/common.h    |  2 +-
>   drivers/vfio/pci/virtio/legacy_io.c | 17 +++---
>   drivers/vfio/pci/virtio/migrate.c   | 90 ++++++++++++-----------------
>   3 files changed, 46 insertions(+), 63 deletions(-)
>