Re: [Qemu-devel] [PATCH 1/1] e820: pass high memory too.

[Gerd Hoffmann <kraxel@redhat.com>]

On Do, 2013-10-17 at 15:00 +0200, Andrea Arcangeli wrote:
> Hi,
> 
> On Thu, Oct 17, 2013 at 01:09:38PM +0200, Gerd Hoffmann wrote:
> > We have a fw_cfg entry to pass e820 entries from qemu to the firmware.
> > Today it's used to pass reservations only.  This patch makes qemu pass
> > entries for RAM too.
> > 
> > This allows to pass RAM sizes larger than 1TB to the firmware and it
> > will also allow to pass non-contignous memory ramges should we decide
> > to implement that some day, say for our virtual numa nodes.
> > 
> > Obviously this needs some extra care to not break existing firware.
> > 
> > SeaBIOS loads the entries and happily adds them without looking at the
> > type.  Which is problematic for memory below 4g as this will overwrite
> > reservations added for bios memory etc.  For memory above 4g it works
> > just fine, seabios will merge the entry derived from cmos with the one
> > loaded from fw_cfg.
> 
> The reason for not fixing the cmos and defer the fixage of the >1TB
> boot, is to develop a better approach, and this mixture of e820 and
> cmos doesn't look like an improvement. The only thing it avoids is to
> touch seabios but it provides no benefit whatsoever if compared to
> fixing the cmos which looks cleaner to me than having to compute a mix
> of cmos and e820 in seabios (and potentially having other bioses
> following this mix-incomplete-API).

e820 allows to pass non-contignous ram ranges to seabios (not that qemu
supports that today, but when implemented the qemu/seabios interface
will deal with it just fine).  How you'll do that with the cmos?

> I thought the reason of deferring the fixage of >1TB boot to wait for
> a better approach and better API, I didn't think the end result had to
> be a mix API that adds no value.

IMO e820 is better than CMOS.

> The premise that "this will also allow to pass non-contiguous memory"
> is partly false, as you can't use the e820 API below 4g so there's no
> way to create non contiguous memory with this mix-cmos-e820-API.

Sure you can.  Why do you think you can't?

> So instead of adding "if (0)" patches and requiring bioses to mix
> information from e820 maps and cmos to boot with more than 1TB, why
> can't simply seabios can be fixed to preserve its own reservations
> (fragmenting the e820 map passed by qemu) while it build the e820 map?
> 
> So then we the qemu API becomes:
> 
>        e820_add_entry(0, ram_size, E820_RAM);

That is the goal.  seabios will be fixed to deal with this correctly.
I don't want break old seabios versions though (especially not before we
have a seabios release which can handle it), so I'll wait with flipping
the switch for that.

cheers,
  Gerd