[SeaBIOS] about [SeaBIOS PATCH] Try to hard-reboot on rerun of post even on emulators.

Kevin O'Connor kevin at koconnor.net
Tue Apr 21 02:29:24 CEST 2015

On Tue, Apr 21, 2015 at 07:31:36AM +0800, Amos Kong wrote:
> Hi Kevin,
> When I use old seabios in some stable linux release, some bootable
> devices (2 ide disks) would be lost when I try to restart guest by
> Ctrl+Alt+Delete during boot stage.
> Releated Red Hat BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1129549
> I found an upstream commit [1] fixed this bug, but when I backport
> this patch to old seabios, guest will shutdown when I try to restart
> by Ctrl+Alt+Delete during boot stage.
> Kevin, can you help to describe that:
> "Unfortunately, kvm does not keep a  pristine copy of the BIOS at 0xffff0000"
> It's a kvm (userspace, QEMU) bug?
> If it's a qemu-kvm bug, I should also fix this bz in old stable release.

Yes, my recollection was that it was a kvm bug.  It was fixed in kvm
after the above was committed to seabios - I don't know what the
commit id was.

On a typical x86 machine, the BIOS image is located in read-only
memory at 0xffff0000.  The chipsets typically also support shadowing
that image to ram (or as a read-only copy) at 0xf0000.  However,
neither qemu nor kvm fully support all the shadowing capabilities of a
typical x86 chipset.  So, seabios will copy itself from the image at
0xffff0000 to ram at 0xf0000.  Unfortunately, kvm had a bug where the
resulting ram image at 0xf0000 was actually mapped to the same ram at
0xffff0000 and changes to the memory copy at 0xf0000 would also change
the copy at 0xffff0000.  This made it impossible for reboots to
redeploy the original pristine copy of seabios.


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