Re: [PATCH v2] uio_hv_generic: Let userspace take care of interrupt mask

[Stephen Hemminger <stephen@networkplumber.org>]

On Thu, 28 Aug 2025 10:12:00 +0530
Naman Jain <namjain@linux.microsoft.com> wrote:

> Remove the logic to set interrupt mask by default in uio_hv_generic
> driver as the interrupt mask value is supposed to be controlled
> completely by the user space. If the mask bit gets changed
> by the driver, concurrently with user mode operating on the ring,
> the mask bit may be set when it is supposed to be clear, and the
> user-mode driver will miss an interrupt which will cause a hang.
> 
> For eg- when the driver sets inbound ring buffer interrupt mask to 1,
> the host does not interrupt the guest on the UIO VMBus channel.
> However, setting the mask does not prevent the host from putting a
> message in the inbound ring buffer. So let’s assume that happens,
> the host puts a message into the ring buffer but does not interrupt.
> 
> Subsequently, the user space code in the guest sets the inbound ring
> buffer interrupt mask to 0, saying “Hey, I’m ready for interrupts”.
> User space code then calls pread() to wait for an interrupt.
> Then one of two things happens:
> 
> * The host never sends another message. So the pread() waits forever.
> * The host does send another message. But because there’s already a
>   message in the ring buffer, it doesn’t generate an interrupt.
>   This is the correct behavior, because the host should only send an
>   interrupt when the inbound ring buffer transitions from empty to
>   not-empty. Adding an additional message to a ring buffer that is not
>   empty is not supposed to generate an interrupt on the guest.
>   Since the guest is waiting in pread() and not removing messages from
>   the ring buffer, the pread() waits forever.
> 
> This could be easily reproduced in hv_fcopy_uio_daemon if we delay
> setting interrupt mask to 0.
> 
> Similarly if hv_uio_channel_cb() sets the interrupt_mask to 1,
> there’s a race condition. Once user space empties the inbound ring
> buffer, but before user space sets interrupt_mask to 0, the host could
> put another message in the ring buffer but it wouldn’t interrupt.
> Then the next pread() would hang.
> 
> Fix these by removing all instances where interrupt_mask is changed,
> while keeping the one in set_event() unchanged to enable userspace
> control the interrupt mask by writing 0/1 to /dev/uioX.
> 
> Fixes: 95096f2fbd10 ("uio-hv-generic: new userspace i/o driver for VMBus")
> Suggested-by: John Starks <jostarks@microsoft.com>
> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
> Cc: <stable@vger.kernel.org>

Makes sense. I think the logic got carried over from uio.
Does it need to make sure interrupt is masked by default to avoid
races at startup?