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Re: [PATCH for-4.22] char/ns16550: bound execution time of ns16550_interrupt()
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 23 Jun 2026 16:19:08 +0200
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- Cc: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 23 Jun 2026 14:19:21 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Jun 23, 2026 at 03:46:11PM +0200, Jan Beulich wrote:
> On 23.06.2026 15:36, Oleksii Kurochko wrote:
> > On 6/23/26 12:31 PM, Roger Pau Monne wrote:
> >> The current logic in ns16550_interrupt() will loop until the device sets
> >> the NOINT in IIR. At least on the Lenovo ThinkSystem SR630 V4 the flow
> >> control of the serial-over-lan emulated UART seems to be broken, as it
> >> doesn't set the NOINT bit consistently. The Transmitter Holding Register
> >> Empty in LSR also seems to not be properly signaled, as even with it set
> >> writes to the transmit register take ~6ms. This leads to the watchdog
> >> triggering very easily on such system.
> >>
> >> Introduce an upper bound on the execution time of ns16550_interrupt(), this
> >> is currently set as 4x the polling interval, which is calculated as the
> >> time to fill RX FIFO and/or empty TX FIFO. The current maximum is 5ms.
> >> Once the timeout triggers the interrupt is disabled and the uart is
> >> switched to polling mode.
> >
> > Don't you mmiss Fixes: tag?
>
> Fixes: "SoL on Lenovo ThinkSystem SR630 V4"
>
> you mean? I think there's nothing wrong with our pre-existing code, and
> the changes here instead are a workaround for some (apparently) badly
> implemented SoL.
It was on purpose that no Fixes tag was provided.
Xen code would be fine with well-behaved uarts, however most of the
serial-over-lan emulated ones are not well behaved it seems.
There's a possible issue with the unbounded loop in
ns16550_interrupt() as it's relying solely on hardware register values
to terminate, which again would be OK if hardware was correctly
implemented. I don't think this warrants a Fixes tag.
Thanks, Roger.
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