[SeaBIOS] [RFC PATCH 8/9] pc: adjust e820 map on hot-add and hot-remove

Vasilis Liaskovitis vasilis.liaskovitis at profitbricks.com
Mon Apr 23 13:27:40 CEST 2012

On Sun, Apr 22, 2012 at 04:58:47PM +0300, Gleb Natapov wrote:
> On Thu, Apr 19, 2012 at 04:08:46PM +0200, Vasilis Liaskovitis wrote:
> >  Hotplugged memory is not persistent in the e820 memory maps. After hotplugging
> >  a memslot and rebooting the VM, the hotplugged device is not present.
> > 
> >  A possible solution is to add an e820 for the new memslot in the acpi_piix4
> >  hot-add handler. On a reset, Seabios (see next patch in series) will enable all
> >  memory devices for which it finds an e820 entry that covers the devices's address
> >  range.
> > 
> >  On hot-remove, the acpi_piix4 handler will try to remove the e820 entry
> >  corresponding to the device. This will work when no VM reboots happen
> >  between hot-add and hot-remove, but it is not a sufficient solution in
> >  general: Seabios and GuestOS merge adjacent e820 entries on machine reboot,
> >  so the sequence hot-add/ rebootVM / hot-remove will fail to remove a
> >  corresponding e820 entry at the hot-remove phase.
> > 
> Why do you need this path and the next one? Bios can restore the state
> of memslots and build e820 map by reading mems_sts.

i see, that is a simpler solution. Since qemu currently creates most ram e820map
entries and passes them to seabios, I tried to follow the same approach. But
your suggestion makes things easier and we don't have to worry about merged e820
entries on hot-remove.  I 'll rework it.


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