[SeaBIOS] [PATCH 2/2] Boot Linux using QEMU fw_cfg DMA interface

[Kevin O'Connor]

On Thu, Aug 06, 2015 at 02:52:16PM +0200, Marc Marí wrote:
> On Thu, 6 Aug 2015 13:38:33 +0100
> Stefan Hajnoczi <stefanha at gmail.com> wrote:
> 
> > On Thu, Aug 6, 2015 at 12:02 PM, Marc Marí <markmb at redhat.com> wrote:
> > >      case IPL_TYPE_BEV:
> > > -        boot_rom(ie->vector);
> > > +        if (qemu_cfg_dma_enabled()) {
> > > +            boot_linux_cfg_dma();
> > > +        } else {
> > > +            boot_rom(ie->vector);
> > > +        }
> > >          break;
> > 
> > I thought the fw_cfg DMA interface is equivalent to the PIO interface?
> > 
> > Therefore, your Linux loading code completely replaces the
> > linuxboot.rom.  Why would you want to keep using linuxboot.rom in the
> > PIO case if SeaBIOS now has to code to launch the kernel?
> > 
> > By the way, I think this change is incorrect.  We shouldn't hijack the
> > BEV.  Instead fw_cfg should simply add a higher priority IPL entry
> > when the kernel file is present.  That way we just don't execute the
> > option ROM.
> > 
> 
> linuxboot.rom is doing all the work of reading and copying the kernel
> to the correct position through the PIO interface, in assembly. If we
> want to speed up the kernel copying with DMA, it is necessary to change
> it, and implementing fw_cfg DMA in C is a lot easier.
> 
> This new boot is independent of PIO or DMA, so now linuxboot.rom would
> not be necessary. I don't know if this is desirable.
> 
> In any case, I agree with adding a new IPL.

One can look at boot.c:boot_cbfs() (and its call chain) as an example
of how to add a new IPL.

As an aside - if ultimately the goal is to change QEMU so that it no
longer uses linuxboot, I wonder if it makes sense to clean up the
QEMU<->firmware interface for it.  It seems that linuxboot uses 12
old-style fw_cfg entries to pass the kernel.  It seems that could be
replaced with three new-style fw_cfg "file" entries - "/vmlinuz",
"/initrd", and "/cmdline" - and QEMU could avoid having to parse and
manipulate the vmlinuz contents.  I'm guessing all the old-style
fw_cfg entries were implemented to make the linuxboot assembler
easier.  That shouldn't be necessary with SeaBIOS though.  Just
something to think about - it shouldn't be a pre-requisite for the
fw_cfg dma stuff.

-Kevin