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Re: [Qemu-devel] RFH: difference in read-only mapped bios.bin - memory c
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] RFH: difference in read-only mapped bios.bin - memory corruption? |
Date: |
Fri, 18 Aug 2017 09:59:09 +0100 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
* Philipp Hahn (address@hidden) wrote:
> Hello,
>
> Am 15.08.2017 um 13:25 schrieb Laszlo Ersek:
> > On 08/14/17 20:39, Dr. David Alan Gilbert wrote:
> >> * Philipp Hahn (address@hidden) wrote:
> >>> I'm currently investigating a problem, were a Linux VM does not reboot
> >>> and gets stuck in the SeaBIOS reboot code:
> >>>
> >>> I'm using SeaBIOS-1.7 from Debian with a more modern qemu-2.8
> ...>>> If I dump both regions and compare them, I get a difference:
> ...>> You might want seabios commit c68aff5 and b837e6 that got fixed after
> >> I tracked down some reboot hangs - although they were rare, not every
> >> time. c68aff5 did certainly cause a corruption, and the address of that
> >> corruption was determined at link time and could overlay random useful
> >> bits of code if you were unlucky.
>
> Thanks you for the commit IDs - to me this looks like they fixed the
> problem. Testing with seabios-1.10 does not show any reboot problem so far.
>
> >>> 1. How can it be, that the low-mem ROM mapping is modified?
> >>
> >> I can't remember all the details, but PC ROM is shadowed and mapped over
> >> with RAM at various times,
> >
> > Right. I don't remember for sure, but I believe the state of the PAM
> > registers doesn't only affect what the VCPUs see in that address range,
> > but also what your monitor commands will dump. (This would be the
> > logical choice -- make the monitor output what the VCPUs see anyway, at
> > the moment, dependent on the PAM settings.)
>
> That makes sense.
> Do you know by change what change in Qemu triggered that bug, as I've
> never seen any reboot problem with qemu-1.1.2, but only since switching
> to qemu-2.8?
I didn't go back as far as 1.1.2, but I tried bisecting around 2.4/2.6
before I understood the failure and the bisect was very flaky; I think
in the end it's a timing race where it comes down to the exact corrupt
value; going back to ancient qemu might be taking some other path
through seabios but I ddin't investigate.
Dave
> Thanks again for your excellent help.
>
> Philipp
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK