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
