Re: [Xen-devel] [PATCH 00/25 v7] SBSA UART emulation support in Xen

[Bhupinder Thakur <bhupinder.thakur@xxxxxxxxxx>]

Hi Julien,

Thanks for the testing.

On 8 August 2017 at 21:29, Julien Grall <julien.grall@xxxxxxx> wrote:
> Hi Bhupinder,
>
> I gave another and I have a couple of comments.
>
> Booting Linux with earlycon enabled take quite a while. I can see the
> characters coming slower than on the minitel. It seems to be a bit better
> after switching off the bootconsole. Overall Linux is taking ~20 times to
> boot with pl011 vs HVC console.
>
> I do agree that pl011 is emulated and therefore you have to trap after each
> character. But 20 times sounds far too much.
>
I think this slowness could be due to ratelimiting of the pl011 events
in xenconosle. Currently, the rate limit is
set to 30 events per 200 msecs (see RATE_LIMIT_ALLOWANCE/RATE_LIMIT_PERIOD).

I increased the rate limit to 600 events (30 * 20) per 200 msecs. With
this change,
I see that the the find command is running faster and smoother.
Earlier the find output would be jerky.

> After that I tried to stress the emulation a bit with "find ." to get a lot
> of output. And I noticed a lot of message similar to the one below on xen
> console:
>
> d6v0 vpl011: Unexpected OUT ring buffer full
>
> Associated to that the character have been eaten resulting to non-sense log.
>
> A bit above the printk printing this message, there are a comment saying:
>
>     /*
>      * It is expected that the ring is not full when this function is called
>      * as the guest is expected to write to the data register only when the
>      * TXFF flag is not set.
>      * In case the guest does write even when the TXFF flag is set then the
>      * data will be silently dropped.
>      */
>
> I am quite surprised that Linux is not looking at the TXFF flags. So this
> needs some investigation.

I ran 'find' but could not reproduce the issue.

I will try to reproduce this issue by reducing the OUT buffer size.

Regards,
Bhupinder

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