Re: lost serial characters on boot

[Frediano Ziglio <freddy77@xxxxxxxxx>]

On Fri, 17 Oct 2025, 18:16 Andrew Cooper, <andrew.cooper3@xxxxxxxxxx> wrote:

Hi,

I think we've got a bug with the serial console.  I'm reliably loosing a
small part of the boot message. e.g.:

(XEN) Latest ChangeSet: Fri Oct 17 17:24:59 2025 git:5e79cfcd2100
(XEN) build-id: d1536ff6698b3e3ba307d551abf
 Family 25 (0x19 Stepping 1 (raw 00a00f11)
(XEN) BSP microcode revision: 0x0a001137

whereas from xl dmesg after the system boots, we see things correctly:

(XEN) Latest ChangeSet: Fri Oct 17 17:24:59 2025 git:5e79cfcd2100
(XEN) build-id: d1536ff6698b3b8968e883fc39e3ba307d551abf
(XEN) CPU Vendor: AMD, Family 25 (0x19), Model 1 (0x1), Stepping 1 (raw
00a00f11)
(XEN) BSP microcode revision: 0x0a001137


The lost bit of text differs even when rebooting the same Xen binary,
but it's always around this area, very early on boot.

I've been seeing this for a little while, but one of my common dev
machines is showing it fairly reliably at the moment.

Checking a transcript of the terminal, we're missing characters, rather
than having additional control characters causing the corruption.

I suppose I need to stare at console_init_preirq() some more, but if
anyone has ideas, I'm all ears.

~Andrew

Hi,

   I would guess the code for early boot is different and is not waiting correctly if the serial is able to accept new characters so the hardware buffer is overwritten.

Frediano